test(ci): load the shared bats helper in the labeler contract test

Every unit bats file under hack/ must load test_helper so it runs with
nounset, and hack/bats-strict-setup.bats fails the suite for a file
that does not.

Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>

hack/label-workflows-contract.bats

@@ -10,6 +10,8 @@
 # mapping demoted to a YAML `#` or JavaScript `//` comment must never satisfy
 # the contract. See hack/promote-gate-contract.bats for the pattern.
 
+load test_helper
+
 REPO_ROOT="$(cd "$(dirname "${BATS_TEST_FILENAME:-$0}")/.." && pwd)"
 PR_LABELER="$REPO_ROOT/.github/workflows/pr-labeler.yaml"
 STALE="$REPO_ROOT/.github/workflows/stale.yaml"