fix: use correct env var to detect gitlab CI for OIDC

[smoores-dev]

What's the problem this PR addresses?

When #6898 added support for OIDC publishing, it copied the getOidcToken function from npm's implementation. However, ciInfo was directly replaced with process.env, which coincidentally worked fine for GitHub Actions (both the ciInfo constant and the environment variable are named GITHUB_ACTIONS), but not for GitLab (the ciInfo constant is GITLAB, but the actual env var is GITLAB_CI.

How did you fix it?

Replaced process.env.GITLAB with process.env.GITLAB_CI

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

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