specify rust stable toolchain

.github/workflows/rust.yml

@@ -15,6 +15,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          override: true
       - name: Build
         run: cargo build --verbose
       - name: Run tests