use nightly rust in github actions

.github/workflows/rust.yml

@@ -11,11 +11,13 @@ env:
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
-
     steps:
       - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
       - name: Build
         run: cargo build --verbose
       - name: Run tests