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.