Parameterize nightly toolchain in CI (#5330)

# Objective

Rust's nightly builds semi-regularly break us (or our dependencies). This creates churn and angst when we're just trying to get our jobs done.

We do still want nightly builds for a variety of reasons:

* cargo-udeps requires nightly and likely always will.
* Helps us catch rust nightly bugs quickly. We're "good citizens" if we regularly report regressions.
* Lets us prepare for "actual expected breakage" ahead of stable releases so we avoid breaking main users.

## Solution

* This pr parameterizes the nightly toolchain, making it an easy one-liner to pin our builds to a specific nightly, when required.
* Put nightly jobs to check wasm and nightly on their own matrix. I also removed tests on nightly linux, just build check
* alternative to https://github.com/bevyengine/bevy/pull/5329

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

.github/bors.toml

@@ -1,10 +1,10 @@
 status = [
-    "build (stable, windows-latest)",
-    "build (stable, ubuntu-latest)",
-    "build (stable, macos-latest)",
-    "build (nightly, ubuntu-latest)",
-    "build-wasm (stable, ubuntu-latest)",
-    "build-wasm (nightly, ubuntu-latest)",
+    "build (windows-latest)",
+    "build (ubuntu-latest)",
+    "build (macos-latest)",
+    "build-wasm",
+    "nightly-checks (wasm32-unknown-unknown)",
+    "nightly-checks (x86_64-unknown-linux-gnu)",
     "build-android",
     "markdownlint",
     "run-examples",

.github/workflows/ci.yml

@@ -9,18 +9,13 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  NIGHTLY_TOOLCHAIN: nightly-2022-07-13
 
 jobs:
   build:
     strategy:
       matrix:
-        toolchain: [stable, nightly]
         os: [windows-latest, ubuntu-latest, macos-latest]
-        exclude:
-          - os: macos-latest
-            toolchain: nightly
-          - os: windows-latest
-            toolchain: nightly
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3
@@ -32,11 +27,7 @@ jobs:
             ~/.cargo/registry/cache/
             ~/.cargo/git/db/
             target/
-          key: ${{ runner.os }}-cargo-build-${{ matrix.toolchain }}-${{ hashFiles('**/Cargo.toml') }}
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: ${{ matrix.toolchain }}
-          override: true
+          key: ${{ runner.os }}-cargo-build-stable-${{ hashFiles('**/Cargo.toml') }}
       - name: Install alsa and udev
         run: sudo apt-get update; sudo apt-get install --no-install-recommends libasound2-dev libudev-dev
         if: runner.os == 'linux'
@@ -47,6 +38,35 @@ jobs:
           CARGO_INCREMENTAL: 0
           RUSTFLAGS: "-C debuginfo=0 -D warnings"
 
+  nightly-checks:
+    strategy:
+      matrix:
+        target: [wasm32-unknown-unknown, x86_64-unknown-linux-gnu]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: ${{ env.NIGHTLY_TOOLCHAIN }}
+          target: ${{ matrix.target }}
+          override: true
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ubuntu-latest-cargo-build-stable-${{ matrix.target }}-${{ hashFiles('**/Cargo.toml') }}
+      - name: Install alsa and udev
+        run: sudo apt-get update; sudo apt-get install --no-install-recommends libasound2-dev libudev-dev
+      - name: Check wasm
+        uses: actions-rs/cargo@v1
+        with:
+          command: check
+          args: --target ${{ matrix.target }}
+
   ci:
     runs-on: ubuntu-latest
     steps:
@@ -87,7 +107,7 @@ jobs:
           key: ${{ runner.os }}-cargo-miri-${{ hashFiles('**/Cargo.toml') }}
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: ${{ env.NIGHTLY_TOOLCHAIN }}
           components: miri
           override: true
       - name: Install alsa and udev
@@ -132,11 +152,7 @@ jobs:
         run: cargo run -p ci -- compile
 
   build-wasm:
-    strategy:
-      matrix:
-        toolchain: [stable, nightly]
-        os: [ubuntu-latest]
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     needs: build
     steps:
       - uses: actions/checkout@v3
@@ -148,17 +164,17 @@ jobs:
             ~/.cargo/registry/cache/
             ~/.cargo/git/db/
             target/
-          key: ${{ runner.os }}-cargo-build-wasm-${{ matrix.toolchain }}-${{ hashFiles('**/Cargo.toml') }}
+          key: ubuntu-assets-cargo-build-wasm-stable-${{ hashFiles('**/Cargo.toml') }}
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: ${{ matrix.toolchain }}
+          toolchain: stable
           target: wasm32-unknown-unknown
           override: true
       - name: Check wasm
         uses: actions-rs/cargo@v1
         with:
           command: check
-          args: --target wasm32-unknown-unknown --no-default-features --features bevy_winit,x11,hdr,bevy_gltf
+          args: --target wasm32-unknown-unknown
 
   markdownlint:
     runs-on: ubuntu-latest
@@ -291,7 +307,7 @@ jobs:
           key: ${{ runner.os }}-cargo-check-unused-dependencies-${{ hashFiles('**/Cargo.toml') }}
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: ${{ env.NIGHTLY_TOOLCHAIN }}
           override: true
       - name: Installs cargo-udeps
         run: cargo install --force cargo-udeps