CI: Only run OCI workflow for branches on main repository

This workflow fails for PRs created from outside branches because the
runner doesn't have access to secrets from the main repository, so the
runner can't authenticate with docker.

.github/workflows/rabbitmq-oci.yaml

@@ -14,6 +14,10 @@ concurrency:
   cancel-in-progress: true
 jobs:
   build-package-generic-unix:
+    # Limit this workflow to branches in the main repository.
+    # Forks are unable to run this workflow because they don't
+    # have access to the docker login credentials.
+    if: github.repository == 'rabbitmq/ra'
     runs-on: ubuntu-latest
     outputs:
       TAG_1: ${{ steps.tags.outputs.TAG_1 }}