Prefer `bundle pristine` over `gem pristine`

What does this merge request do and why?

bundle pristine uses the bundle config options to build gems, while gem pristine does not.

This is an important distinction for building re2 against libre2 2023-07-01. re2 v1.6.0 doesn't build without the special flags added in !3226 (merged).

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.