Set GitLab staging customer portal as default

What does this merge request do and why?

This MR sets the following environment variables by default when GDK starts, instead of manually setting them in the env.runit:

export GITLAB_LICENSE_MODE=test
export CUSTOMER_PORTAL_URL=https://customers.staging.gitlab.com

Closes #1824 (closed)

How to set up and validate locally

1. Ensure there's no license already set up in your GDK.
2. Check out the main branch - git checkout main.
3. Restart your GDK - gdk restart.
4. Try adding a license - you will see an error saying that the license key is invalid.
5. Check out this branch by git checkout 1824-set-staging-customer-portal-as-default.
6. Restart your GDK - gdk restart.
7. Add a license again - this time it should succeed.

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.