jwt: replace deprecated `CI_JOB_JWT` variable with an ID token
See https://docs.gitlab.com/ee/update/deprecations.html#old-versions-of-json-web-tokens-are-deprecated
Second attemp for a241d213. I had to revert the original commit to prevent pipelines of other projects failing. The JWT authorizer on the docker registry hosts hadn't been restarted and was still running the old code
Bug: T337474