fix: set sh as default shell for containers (#853)

* fix: set default shell for containers to sh

This matches GitHub Actions behaviour where,
runners use bash since it's guaranteed to exist there
while containers use sh for compatibility

* tests: fixup for default shell

pkg/runner/testdata/shells/pwsh/push.yml

@@ -6,3 +6,10 @@ jobs:
       - shell: pwsh
         run: |
           $PSVersionTable
+  check-container:
+    runs-on: ubuntu-latest
+    container: ghcr.io/justingrote/act-pwsh:latest
+    steps:
+      - shell: pwsh
+        run: |
+          $PSVersionTable