CI: use bash shell even when building on Windows

Author: ctrueden

.github/workflows/build.yml

@@ -30,6 +30,7 @@ jobs:
         run: .github/setup.sh
       - name: Execute the build
         run: .github/build.sh
+        shell: bash
         env:
           GPG_KEY_NAME: ${{ secrets.GPG_KEY_NAME }}
           GPG_PASSPHRASE: ${{ secrets.GPG_PASSPHRASE }}