nixos/git-daemon: only create git user if it will be used #81118
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Motivation for this change
This was broken in 1d61efb, but it doesn't make sense to create the git user iff it will not be used.
A fix for this regression needs to be backported to 20.03.
I added a test that passes with this PR, and would fail without it. It's my first test, let me know if I did something wrong.