repo-updater: always wrap context.Background with internal actor

Created by: unknwon

repo-updater used to have god-view of the entire repo table, but with the merge of underlying DB logic, the repoStore it is using now respecting the repository permissions, which makes the repo-updater not able to see the private repos anymore.

This causes two symptoms (maybe there are more):

1. Permissions syncing stops working because it can't see private repos (no leakage, instead non-admin could see nothing).
2. Site admin can still visit the private repo on Sourcegraph but not able to view the settings page, because the page sends a request to the repo-updater and the latter couldn't see it.

I found this bug in my personal instance, triggered by my recent update to insider version, and my friend couldn't see my repo anymore