Fix Github Actions for recent system changes

Author: alexcrichton

.github/workflows/main.yml

@@ -29,6 +29,7 @@ jobs:
     - name: Install Rust (rustup)
       run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
       if: matrix.os != 'macos-latest'
+      shell: bash
     - name: Install Rust (macos)
       run: |
         curl https://sh.rustup.rs | sh -s -- -y