Skip asdf specific preflight check if asdf.opt_out

What does this merge request do and why?

Fixes preflight checks failing :

$ make platform-update
INFO: Performing common preflight checks..
ERROR: RVM or rbenv detected, which can cause issues with asdf.
INFO: We recommend you uninstall RVM or rbenv, or remove RVM or rbenv from your PATH variable.
INFO: For more information, see https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/migrate_to_asdf.md.
make[1]: *** [platform-update-run] Error 1
make: *** [platform-update-timed] Error 2

With this, it now works:

$ make platform-update
INFO: Performing common preflight checks..

--------------------------------------------------------------------------------
Ensuring platform software installed
--------------------------------------------------------------------------------
INFO: Setting up 'darwin' platform..
INFO: This GDK has already had platform packages installed.
INFO: Remove '/Users/tkuah/code/gdk-ee/.cache/.gdk_platform_setup' to force execution.
INFO: Installing gitlab-development-kit Ruby gem..
Latest version already installed. Done.

Related: !3033 (merged)

How to set up and validate locally

1. Have RVM, or Rbenv installed
2. Set asdf.opt_out to true
3. Run make platform-update, or gdk update

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.