nixos/systemd: migrate systemd-timesync state when required

Somewhen between systemd v239 and v242 upstream decided to no longer run
a few system services with `DyanmicUser=1` but failed to provide a
migration path for all the state those services left behind.

For the case of systemd-timesync the state has to be moved from
/var/lib/private/systemd/timesync to /var/lib/systemd/timesync if
/var/lib/systemd/timesync is currently a symlink.

We only do this if the stateVersion is still below 19.09 to avoid
starting to have an ever growing activation script for (then) ancient
systemd migrations that are no longer required.

See https://github.com/systemd/systemd/issues/12131 for details about
the missing migration path and related discussion.

nixos/modules/system/boot/timesyncd.nix

@@ -40,6 +40,15 @@ with lib;
     users.users.systemd-timesync.uid = config.ids.uids.systemd-timesync;
     users.groups.systemd-timesync.gid = config.ids.gids.systemd-timesync;
 
+    system.activationScripts.systemd-timesyncd-migration = mkIf (versionOlder config.system.stateVersion "19.09") ''
+      # workaround an issue of systemd-timesyncd not starting due to upstream systemd reverting their dynamic users changes
+      #  - https://github.com/NixOS/nixpkgs/pull/61321#issuecomment-492423742
+      #  - https://github.com/systemd/systemd/issues/12131
+      if [ -L /var/lib/systemd/timesync ]; then
+        rm /var/lib/systemd/timesync
+        mv /var/lib/private/systemd/timesync /var/lib/systemd/timesync
+      fi
+    '';
   };
 
 }