add condition to only run in jobs with this variable

although i think we should do this test always
1 job for test-garbled-output in 14 seconds (queued for 5 seconds)
latest
Name Stage Failure
failed
test Test
Processing triggers for ca-certificates (20200601~deb10u2) ...
Updating certificates in /etc/ssl/certs...
0 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d...
done.
$ ./validate-gitlab-yaml.py *.yml
ERROR: missing GITLAB_PRIVATE_TOKEN environment, aborting
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1