pythonPackages.wrf-python: Fix build (#56989)

The latest changes to support better cross-compilation compatibility
have introduced a stricter handling of dependency specification in
python. Since b4acd977, gfortran should be put into nativeBuildInputs,
as it's a build-time only dependency for wrf-python. Similarly, mock is
only required when testing, so it should go to checkInputs.

pkgs/development/python-modules/wrf-python/default.nix

@@ -16,13 +16,14 @@ buildPythonPackage rec {
     numpy
     xarray
   ];
-  buildInputs = [
+
+  nativeBuildInputs = [
     gfortran
-  ] ++ lib.optional (pythonOlder "3.3") mock;
-  
+  ];
+
   checkInputs = [
     netcdf4
-  ];
+  ] ++ lib.optional (pythonOlder "3.3") mock;
 
   doCheck = true;
   checkPhase = ''