[gitlab:check] Fix detecting the status of sidekiq

If you are running another sidekiq instance on your server, e.g. GitLab
CI, the check script would parse the output of `ps aux` searching for
`sidekiq` and returning success, although the GitLab sidekiq may not be
running.

Now the `ps` call will only print the processes run by the GitLab user.
Merge pull request #5191 from jojosch/fix-check-script-sidekiq

[gitlab:check] Fix detecting the status of sidekiq