Warn if GDK is not yet setup in Remote Development

What does this merge request do and why?

This MR emits a ⚠️ message on each GDK comand when GDK is run in Workspaces and not yet setup.

⚠️  WARNING: GDK setup in progress...
Run `tail -f /projects/workspace-logs/poststart-stdout.log`

How to set up and validate locally

1. gdk help - no warning
2. GL_WORKSPACE_DOMAIN_TEMPLATE=1 - ⚠️
3. touch <gdk_root>/.cache/.gdk_setup_complete
4. gdk help - no warning

Closes https://gitlab.com/gitlab-org/gitlab-development-kit/-/issues/2828

How to set up and validate in Workspaces

1. Create a workspace via Code > Open in Workspace
2. Add envvar GDK_SELF_UPDATE 0
3. Create workspace
4. ⏳ wait for boot
5. Open workspace
6. Run gdk version

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 MR references an issue describing the change.
• 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.