CI: `review_specs` jobs failing, Runner registration failure

Summary

Noticed in !3669 (comment 1901141822) that all of the review_specs jobs were failing:

ERROR: Registering runner... failed runner=AOeXHZij status=POST https://gitlab-gke126-review-e6e-5kh5n4.cloud-native-v126.helm-charts.win/api/v4/runners: 500 Internal Server Error ()

Noted that this appears to be happening in other MR pipelines as well, such as https://gitlab.com/gitlab-org/charts/gitlab/-/pipelines/1286170684.

Acceptance criteria

• Root cause is found
• review_specs jobs pass