The regression was introduced by:
1b8f52d920
I did that because there's a test specifying that a shared runner cannot
be enabled, in the API. So I assume that is the case for non-admin, but
admins should be able to do so anyway.
Also added a test to make sure this won't regress again.
Closes#19039
* master:
Move CI admin builds and runners specs to correct directory
Fix 500 when viewing specific runners on runners page
Fix Ci::Project migration not migrating columns that cannot be NULL
Fix MySQL migration of CI emails
Minor fix in flow 'Merge when build succeeds'