Ensure Bunder.with_unbundled_env is used for preflight-checks

What does this merge request do and why?

For anyone not using asdf, our support/bundle-exec approach doesn't fully unload Bundler, which creates a lot of incompatibilities when running code in gitlab folder from gdk one.

This ensure we are unloading Bundler before running the command, when Bundler is loaded.

How to set up and validate locally

In a machine that is setup using RVM/HomeBrew:

Before the change:

$ rake preflight-checks

ERROR: The following Ruby Gems appear to have issues:

ffi

Try running the following to fix:

cd ...gdk-dev/gitlab && bundle pristine ffi

# exit 1
$

after (current branch):

$ rake preflight-checks

# exit 0
$

Fix #2039 (closed)

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.