Install GDK gems into project path

What does this merge request do and why?

Make bundle install leave gems in project's .gems/ directory. This dir won't be reset on Gitpod restart, because it is located under /workspace.

How to set up and validate locally

1. Open GDK repo in Gitpod
2. Run bundle install
3. Run gp stop and resume the workspace
4. Run bundle install again and see it reuses existing gems

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.