Due to a change in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24245, the Detect
Host Key feature in the SSH mirroring stopped working.
`SshHostKey#primary_key` was being used instead of the hard-coded
`:id`. However, `SshHostKey#find_by` was expecting the symbolized `:id`
rather than the string `id`, so it could never find the host key it was
supposed to update.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56855