Praefect check subcommand bypass with PRAEFECT_CHECK_SKIP_CHECKS env var

If the praefect check subcommand sees PRAEFECT_CHECK_SKIP_CHECKS set to true, simply return a 0 and do not proceed to run all the checks. Omnibus will set this environment variable so that the admin can do

gitlab-ctl start praefect --skip-checks

The issue for the omnibus change is here: omnibus-gitlab#6464 (closed)