`package-and-qa` fails in master because it expects an MR

Job #1757249675 failed for c9eb99bd:

$ tooling/bin/find_change_diffs ${CHANGES_DIFFS_DIR}
tooling/bin/find_change_diffs:11:in `fetch': key not found: "CI_MERGE_REQUEST_PROJECT_PATH" (KeyError)
	from tooling/bin/find_change_diffs:11:in `<main>'

We should skip that script when the pipeline isn't run in an MR.