Dynamically select gitlab repo remote in git fetch

What does this merge request do and why?

This MR fixes that the gitlab-org/gitlab> remote name is hard-coded for git fetch during gdk update by using the repository URL from the config.

If the developer is in a network situation where gitlab.com is not accessible over HTTPS but over SSH or another URL, they can change this URL and probably have with gdk install gitlab_repo=....

Partially addresses #2146

How to set up and validate locally

1. Go to /gitlab
2. Rename your remote to something other than origin, e.g. git remote rename origin upstream
3. Run GDK_SELF_UPDATE=0 gdk update
4. See that it updates the ./gitlab directory

And finally, maybe change your remote name back if you’re used to it being origin.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.