Clear HOMEBREW_BUNDLE_FILE for GDK environment

What does this merge request do and why?

If a user sets HOMEBREW_BUNDLE_FILE to some other file, then the GDK will use that file instead of its local one. Clear out this variable to prevent make bootstrap from using the wrong file.

Relates to #1965 (closed)

How to set up and validate locally

1. Open a terminal
2. cd $HOME
3. echo 'cask "kitty"' > $HOME/Brewfile
4. export HOMEBREW_BUNDLE_FILE="$HOME/Brewfile"
5. git clone ``https://gitlab.com/gitlab-org/gitlab-development-kit.git
6. cd ./gitlab-development-toolkit
7. make bootstrap

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.