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.

editor/plugins/SCsub

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
 
 Import('env')
-Export('env')
+
 env.add_source_files(env.editor_sources, "*.cpp")