Make sure failed builds result in an error on Windows

By default, this was run on PowerShell on Windows, and even though the
Actions environment is supposed to immediately exit on failures[1], this
doesn't seem to have effect.

Switching to bash for unified behavior on all platforms.

[1]: https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference

.github/workflows/go.yml

@@ -21,6 +21,7 @@ jobs:
         run: go mod verify
 
       - name: Build
+        shell: bash
         run: |
           go test ./...
           go build -v ./cmd/gh