Fixes a regression caused by a5ee39266c.

If the user id and group id were different than 991:991
(which used to be a hardcoded default for us long ago),
there was a mismatch between what Synapse was trying to use (991:991)
and what it was actually started with (in `--user=..`). It was then
trying to change ownership, which was failing.

This was mostly affecting newer installations which were not using the
991:991 defaults we had long ago (since a1c5a197a9).