Retry merge commit retrieval on failure

Sometimes when merge commit retrieval fails (because of race condition
most likely as pull request event is no ready yet) build-images builds
all images because selective check does not know the merge commit (in
pull request target vuild image failure). That was the only side effect.
because otherwise build image had a fallback to
github.event.pull_request.head.sha in this case.

With this PR, we will retry it and fallback explicitly to
github.event.pull_request.head.sha if it does not work.

.github/workflows/build-images.yml

@@ -91,8 +91,25 @@ jobs:
       - name: Discover PR merge commit
         id: discover-pr-merge-commit
         run: |
+          # Sometimes target-commit-sha cannot be
           TARGET_COMMIT_SHA="$(gh api '${{ github.event.pull_request.url }}' --jq .merge_commit_sha)"
-          echo "TARGET_COMMIT_SHA=$TARGET_COMMIT_SHA" >> ${GITHUB_ENV}
+          if [[ ${TARGET_COMMIT_SHA} == "" ]]; then
+            # Sometimes retrieving the merge commit SHA from PR fails. We retry it once. Otherwise we
+            # fall-back to github.event.pull_request.head.sha
+            echo
+            echo "Could not retrieve merge commit SHA from PR, waiting for 3 seconds and retrying."
+            echo
+            sleep 3
+            TARGET_COMMIT_SHA="$(gh api '${{ github.event.pull_request.url }}' --jq .merge_commit_sha)"
+            if [[ ${TARGET_COMMIT_SHA} == "" ]]; then
+              echo
+              echo "Could not retrieve merge commit SHA from PR, falling back to PR head SHA."
+              echo
+              TARGET_COMMIT_SHA="${{ github.event.pull_request.head.sha }}"
+            fi
+          fi
+          echo "TARGET_COMMIT_SHA=${TARGET_COMMIT_SHA}"
+          echo "TARGET_COMMIT_SHA=${TARGET_COMMIT_SHA}" >> ${GITHUB_ENV}
           echo "target-commit-sha=${TARGET_COMMIT_SHA}" >> ${GITHUB_OUTPUT}
         if: github.event_name == 'pull_request_target'
       # The labels in the event aren't updated when re-triggering the job, So lets hit the API to get