Set installed toolchain as default

The docs for the action indicate that unless specified the toolchain will not be used by default.

Author: wiktor-k

.github/workflows/ci.yml

@@ -67,6 +67,7 @@ jobs:
         uses: actions-rs/toolchain@v1
         with:
           toolchain: ${{ matrix.toolchain }}
+          default: true
       - name: Install Rust target
         run: rustup target add ${{ matrix.target }}
       - name: Check source