jool: prepend kernel.makeFlags to makeFlags

pkgs/os-specific/linux/jool/default.nix

@@ -16,13 +16,12 @@ stdenv.mkDerivation {
     sed -e 's@/lib/modules/\$(.*)@${kernel.dev}/lib/modules/${kernel.modDirVersion}@' -i src/mod/*/Makefile
   '';
 
-  buildPhase = ''
-    make -C src/mod
-  '';
+  makeFlags = kernel.makeFlags ++ [
+    "-C src/mod"
+    "INSTALL_MOD_PATH=${placeholder "out"}"
+  ];
 
-  installPhase = ''
-    make -C src/mod modules_install INSTALL_MOD_PATH=$out
-  '';
+  installTargets = "modules_install";
 
   meta = with lib; {
     homepage = "https://www.jool.mx/";