Add support for alternate GDK repo URL

What does this merge request do and why?

Add support for alternate GDK repo URL

At present the GDK repo URL is hardcoded in the install script. Now you can pass it in via GDK_REPO_URL environment variable.

How to set up and validate locally

curl https://gitlab.com/gitlab-community/gitlab-development-kit/-/raw/support-alt-gdk-repo-url/support/install | GDK_REPO_URL=https://gitlab.com/gitlab-community/gitlab-development-kit.git GDK_CLONE_BRANCH=support-alt-gdk-repo-url bash

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.