Update marlin.py to fix compile error

Replaces:
    for define in env['CPPDEFINES']:

with:
    envdefs = env['CPPDEFINES'].copy()
    for define in envdefs:

in buildroot\share\PlatformIO\scripts\marlin.py
on extui branch

buildroot/share/PlatformIO/scripts/marlin.py

@@ -19,7 +19,8 @@ def copytree(src, dst, symlinks=False, ignore=None):
             shutil.copy2(s, d)
 
 def replace_define(field, value):
-	for define in env['CPPDEFINES']:
+	envdefs = env['CPPDEFINES'].copy()
+    for define in envdefs:
 		if define[0] == field:
 			env['CPPDEFINES'].remove(define)
 	env['CPPDEFINES'].append((field, value))