Since populating the fork networks was scheduled multiple times
because of bugs that needed to be fixed:

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15595/
  Creating fork networks for projects that were missing the root of
  the fork network.

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15709
  The API allowed creating forked_project_links without
  fork_network_members.

Scheduling this migration multiple times would case it to run
concurrently. Which in turn would try to insert the same data into
`fork_network_members` causing duplicate key errors.

This avoids running the migration multiple times.