Make `CI_COMMIT_REF_NAME` and `SLUG` variable idempotent

What does this MR do?

This bug happens only when users use Pipelines for merge requests feature.

Due to the nature of pipelines for merge requests, CI_COMMIT_REF_NAME and CI_COMMIT_REF_SLUG have a merge request ref (refs/merge-requests/:iid/head or /merge) instead of a branch ref. This disturbs users to rely on this variable for Review App or dynamic deployment.

We fix this problem by always setting generic ref name to these variables. With merge request pipelines, it returns a source branch ref instead of a merge request ref.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60822

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team