Fix package helper uses relative path for storing cached commit sha

What does this merge request do and why?

This fixes that the package helper uses a relative path for storing the cached commit SHA. It should use the absolute path, or else it causes gdk update to fail in subdirectories.

How to set up and validate locally

1. Run cd gitlab
2. Run GDK_SELF_UPDATE=0 gdk update
3. See that it works

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.