SCons: Build thirdparty code in own env, disable warnings

Also remove unnecessary `Export('env')` in other SCsubs,
Export should only be used when exporting *new* objects.

modules/gdnative/pluginscript/SCsub

@@ -1,9 +1,6 @@
 #!/usr/bin/env python
 
 Import('env')
-Import('env_modules')
+Import('env_gdnative')
 
-env_pluginscript = env_modules.Clone()
-
-env_pluginscript.Append(CPPPATH=['#modules/gdnative/include/'])
-env_pluginscript.add_source_files(env.modules_sources, '*.cpp')
+env_gdnative.add_source_files(env.modules_sources, '*.cpp')