GH actions: maybe parameterize the rust toolchain

Author: wez

.github/workflows/linux.yml

@@ -14,11 +14,12 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-16.04, ubuntu-18.04]
+        rust_toolchain: [stable, nightly, beta]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1
     - name: Install Rust
-      run: curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain nightly
+      run: curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain ${{ matrix.rust_toolchain }}
     - name: Build and test
       run: |
         source $HOME/.cargo/env