Pre-filled variables do not work for external custom CI config file paths

Summary

Pre-filled variables do not show up on the run pipeline form when the project's CI configuration resides on a separate project.

Steps to reproduce

1. Create project X
2. In project X create a .gitlab-ci.yml
3. In the above file, add pre-defined variables
4. Try to run manual pipeline on project X
5. Pre-filled variables are shown on UI
6. Create project Y
7. Use settings of project Y to point its CI config path to project X: X@.gitlab-ci.yml
8. Try to run a manual pipeline on project Y
9. Pre-filled variables are not shown on the UI

Example Project

https://gitlab.com/gitlab-gold/hchouraria/custom-ci-config-path-proj

What is the current bug behavior?

Pre-filled variables defined in custom CI config path outside of repository does not render in run pipeline form

What is the expected correct behavior?

Pre-filled variables defined in custom CI config path outside of repository render in run pipeline form

Relevant logs and/or screenshots

Issue appears to be with the implementation of the ci_config_for(sha) method. It returns nil when the path is outside of repository.

# p is the project where pipelines need to run
irb(main):007:0> p.ci_config_path
=> ".gitlab-ci.yml@neo/gradle-proj"
irb(main):008:0> p.ci_config_for("master")
=> nil
irb(main):009:0> p.ci_config_for("main")
=> nil
irb(main):010:0> p.ci_config_for("blah")
=> nil
irb(main):011:0> p.ci_config_for("85a2533964185560237c98c80857c1bac7267542")
=> nil

Output of checks

This bug happens on GitLab.com

Possible fixes

Call chain where this fails to resolve:

• https://gitlab.com/gitlab-org/gitlab/blob/7a1b3d9f7491a97160e053b4a96c62cb59d09a10/app/services/ci/list_config_variables_service.rb#L25
• https://gitlab.com/gitlab-org/gitlab/blob/aeff2cf9f841076ee6a1e2bb3d45a1689e38205a/app/models/project.rb#L2586-2588
• https://gitlab.com/gitlab-org/gitlab/blob/f16420e7e65c95032690584a73b5540a2f1885bd/app/models/repository.rb#L1042-1044