Use project CI config path to show CI blob viewer

What does this MR do and why?

When we check for a project's gitlab_ci_yml, we always assume it uses .gitlab-ci.yml. However, a project can have its own custom path for this.

In this commit, we are fixing this for blob viewer.

Related to #26169

Screenshots or screen recordings

With a custom YAML file;

Before this change;

After this change;

MR acceptance checklist

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