Add env for cleanup to skip removing asdf software

What does this merge request do and why?

I often want to use gdk cleanup to remove log files etc associated with running GitLab via GDK.
I however also use many versions of software via asdf, and often don't want to remove all of those additional versions.

The current format of gdk cleanup requires that if I want to remove logs - I also have to remove asdf software as well.

This MR adds an optional ENV GDK_CLEANUP_SOFTWARE, defaulted to true to preserve current behaviour, which can be used to skip the removal of asdf software.

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.