Allow force overwrite of gitlab-shell secret

What does this Merge Request do?

This MR fixes an issue with Gitpod where the gitlab-shell secret is already in place:

--snip--
ln: failed to create symbolic link 'gitlab-shell/.gitlab_shell_secret': File exists
make: *** [Makefile:360: gitlab-shell/.gitlab_shell_secret] Error 1
❌️ ERROR: Failed to reconfigure.
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You can try the following that may be of assistance:

- Run 'gdk doctor'.
- Visit the troubleshooting documentation:
  https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/troubleshooting.md.
- Visit https://gitlab.com/gitlab-org/gitlab-development-kit/-/issues to
  see if there are known issues.
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Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.