CI: Limit job parallelism

We are seeing random CI failures with:

    Failed to set execute bit on remote files

Limit parallelism in case it is related to concurrent access to the same
Docker image.

Author: priteau

.github/workflows/ci.yml

@@ -40,6 +40,7 @@ jobs:
     needs: build
     strategy:
       fail-fast: false
+      max-parallel: 1
       matrix:
         image:
           - 'rockylinux:8.9'