Specify bash shell in copilot-setup-steps job

Add defaults section to specify bash shell for job.

Author: strimo378

.github/workflows/copilot-setup-steps.yml

@@ -13,6 +13,9 @@ jobs:
   # The job must be called copilot-setup-steps for Copilot to detect it.
   copilot-setup-steps:
     runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash
     container:
       image: mcr.microsoft.com/devcontainers/base:ubuntu-24.04
       volumes: