Update bundler config settings in `gdk update` earlier

What does this merge request do and why?

Currently both Gitaly and GitLab projects install Ruby gems, but a gdk update will fail prematurely in the make gitaly-setup step if a C extension library cannot be loaded.

The GitLab project relies on the PostgreSQL directory for the pg gem. We need to make sure that the bundle config settings are correct so that future invocations of gem pristine will work properly. If a user runs rake asdf:uninstall_unnecessary_software and wipes out the old PostgreSQL directory, this will break the pg gem and any subsequent attempts to use Ruby will fail. A gem pristine pg won't fix the issue until the correct PostgreSQL directory gets set.

Relates to #1617

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.