Allow reading from custom CI/CD configuration filename

What does this MR do?

This merge request allows users of gitlab-runner to specify an alternate CI/CD configuration filename (also known as the CI_CONFIG_PATH predefined variable).

Why was this MR needed?

This supports environments where users have a non-default CI/CD configuration filename.

What's the best way to test this MR?

Test coverage should probably be added here; so far I've tested this locally by using make runner-bin-linux to build binaries locally, and then tested those against a repository that contains both a default-named .gitlab-ci.yml file and an alternatively-named CI/CD configuration file.

What are the relevant issue numbers?

Closes #27933

Duplicates / is-equivalent-to !1736 (closed).