jwt: support both `CI_JOB_JWT` and id tokens
Since GitLab was upgraded recently to version 16, this change is a requisite for the pipeline of the following MR to pass: !73 (merged)
Without this change, the usage of an id token precludes the
injection of the CI_JOB_JWT
variable into the pipeline, which in
turn causes the bootstrap-kokkuri-image
job to fail. See also:
https://docs.gitlab.com/ee/update/deprecations.html#old-versions-of-json-web-tokens-are-deprecated
Bug: T337474