Improve how we find the GDK_ROOT file

What does this merge request do and why?

This MR improves how we determine where the GDK_ROOT file is when the gdk command is called. This should address the situation in Gitpod when in the /workspace/gitlab-development-kit/gitlab directory and the gdk command is called but results in:

gitpod /workspace/gitlab-development-kit/gitlab (gdk-gitpod-integration-branch) $ gdk

The current working directory is not inside a gitlab-development-kit
installation. Use 'cd' to go to your gitlab-development-kit or create
a new one with 'gdk init'.

gdk init [DIRECTORY] # Default: /workspace/gitlab/gitlab-development-kit

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.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.