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.)
(cherry picked from commit a774af3605a86719f813ccb80e9f8a5bdef2ee0a)