nix: do not restart sharefs

This avoids disrupting running containers.

Signed-off-by: Ophestra <cat@gensokyo.uk>

nixos.nix

@@ -94,6 +94,10 @@ in
               ]
             } ${cfg.sharefs.name}
         '';
+
+        # do not unmount on configuration changes
+        restartIfChanged = false;
+
         wantedBy = [ "multi-user.target" ];
       };
     };