nixos/nscd: Add requiredBy for the nss targets

This is to ensure the targets are stopped when nscd is stopped to
prevent races on switch. Example interaction: nscd is stopped, some
service that requires nss-user-lookup.target is restarted. Without this
PR, nss-user-lookup.target would still be active, hence the service
would start without nscd running.

nixos/modules/services/system/nscd.nix

@@ -62,6 +62,7 @@ in
         before = [ "nss-lookup.target" "nss-user-lookup.target" ];
         wants = [ "nss-lookup.target" "nss-user-lookup.target" ];
         wantedBy = [ "multi-user.target" ];
+        requiredBy = [ "nss-lookup.target" "nss-user-lookup.target" ];
 
         environment = { LD_LIBRARY_PATH = nssModulesPath; };