The paths in the generated `systemd.pc` were flipped, fix this:
`systemdsystemunitpath`: `/nix/var/nix/profiles/default/lib/systemd/{user => system}`
`systemduserunitpath`: `/nix/var/nix/profiles/default/lib/systemd/{system => user}`
The paths actually used in the code (further below in that patch) were
correct, so keep them there.
Fixes#59473.
It seems nix is much more permissive in applying patches than git am.
These patches were regenerated by running
`git am path/to/nixpkgs/pkgs/os-specific/linux/systemd/*.patch`,
and manually running `patch -p1 < path/to/nixpkgs/pkgs/os-specific/linux/systemd/*N.patch`
where necessary.