CI: Update CNG mirror skip job regex

What does this MR do and why?

Fixes gitlab-org/quality/quality-engineering/team-tasks#4339 (closed)

GitLab CI's =~ operator does partial/substring matching by default. So a regex like /gitlab-go/ matches any job name that contains gitlab-go — including gitlab-gomplate.

When gitlab-go's image already exists in the registry, it gets added to the skip list, building a regex like:

/final-images-listing|gitlab-go|gitaly|.../

This inadvertently also matches gitlab-gomplate and gitaly-init-cgroups, causing those jobs to be skipped with when: never. When those skipped jobs are then needed as dependencies (e.g. gitlab-base depends on gitlab-gomplate), the pipeline fails.

The fix adds ^ (start) and $ (end) anchors with a capturing group:

/^(final-images-listing|gitlab-go|gitaly|...)$/

Now the regex only matches a job name that is exactly one of the listed strings — gitlab-go can no longer accidentally match gitlab-gomplate.

Note: this will need to be backported to fix stable branches

Testing

Tested in draft MR in 18.7 branch https://gitlab.com/gitlab-org/build/CNG-mirror/-/pipelines/2391718585

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