psi-notify: move systemd unit to $out/lib

In $out/share it is NOT picked up into /etc/systemd/... when psi-notify is
added to configuration.systemd.packages .

(Originally put in $out/share b/c a hook copies from $out/lib into there
... so why not just put there? This is why.)

pkgs/applications/misc/psi-notify/default.nix

···
 
     install -D psi-notify $out/bin/psi-notify
     substituteInPlace psi-notify.service --replace psi-notify $out/bin/psi-notify
-    install -D psi-notify.service $out/share/systemd/user/psi-notify.service
+    install -D psi-notify.service $out/lib/systemd/user/psi-notify.service
 
     runHook postInstall
   '';