Fix -Zrandomize-layout setting in Windows jobs

This didn't take effect when run by the default pwsh shell.

Author: dtolnay

.github/workflows/ci.yml

@@ -61,6 +61,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@nightly
       - name: Enable type layout randomization
         run: echo RUSTFLAGS=${RUSTFLAGS}\ -Zrandomize-layout >> $GITHUB_ENV
+        shell: bash
       - run: cargo test
 
   minimal: