System checks should verify `db/schema.rb` is writable
In https://gitlab.com/gitlab-org/gitlab-ce/issues/40480 a user had an issue where the migration check output didn't show anything wrong.
We don't know what caused that, but later, we noticed that their db/schema.rb
file was not writable, even though it is set as such by Omnibus.
We should add a system check to verify that's writable, because otherwise we could get into an unexpected state.