Respect custom gitlab_shell path in gitlab:gitlab_shell:check task

Prior to this change, if I customized the config value of
`gitlab.gitlab_shell.path`, the check would ignore this value in favor
of `"~<gitlab.gitlab_shell.ssh_user>/gitlab-shell/"` resulting in a
failed check when the configuration was otherwise valid.
Merge pull request #6492 from tsigo/gitlab-shell-custom-path

Respect custom gitlab_shell path in gitlab:gitlab_shell:check task