update: Don't update appstream for non-enumerable remotes

That doesn't make any sense to do and is likely to get you
errors.

Closes: #1240
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f