Don't restart systemd-remount-fs

It's only needed during early boot (in fact, it's probably not needed
at all on NixOS). Restarting it is expensive because it does a sync()
of the root file system.

(cherry picked from commit 3b2609deec)

nixos/modules/system/boot/systemd.nix

@@ -814,5 +814,7 @@ in
 
     systemd.services."user@".restartIfChanged = false;
 
+    systemd.services.systemd-remount-fs.restartIfChanged = false;
+
   };
 }