fix(ci): ignore main [skip-build] commits in PR image check

Use git log --first-parent when scanning for [skip-build] so merged main
chore commits do not skip building commit-specific PR tags Prow waits for.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: schultzp2020

.github/actions/check-image-and-changes/action.yaml

@@ -41,9 +41,10 @@ runs:
         echo "Changed files:"
         echo "$CHANGED_FILES"
 
-        # Check for [skip-build] tag in commit messages
+        # Check for [skip-build] on PR commits only. Without --first-parent, merge
+        # refs pull in main chore commits like "update rpms.lock.yaml [skip-build]".
         SKIP_BUILD_TAG=false
-        if git log --format=%B "$BASE_COMMIT"..HEAD | grep -qF "[skip-build]"; then
+        if git log --first-parent --format=%B "$BASE_COMMIT"..HEAD | grep -qF "[skip-build]"; then
           echo "Found [skip-build] tag in commit messages."
           SKIP_BUILD_TAG=true
         fi