ci: run molecule runners sequentially (max-parallel: 1)

With fail-fast: true and max-parallel: 1, runner 0 must complete
before runner 1 starts. If runner 0 fails, runners 1 and 2 are
cancelled. This gives immediate feedback on the first failure
instead of waiting for all 3 to fail in parallel.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.gitea/workflows/ci.yml

@@ -31,6 +31,7 @@ jobs:
     runs-on: docker
     strategy:
       fail-fast: true
+      max-parallel: 1
       matrix:
         runner-index: [0, 1, 2]
     steps: