SCons: Use gnu++20 in metal driver

drivers/metal/SCsub

@@ -36,7 +36,7 @@ env_metal.drivers_sources += thirdparty_obj
 # Enable C++20 for the Objective-C++ Metal code, which uses C++20 concepts.
 if "-std=gnu++17" in env_metal["CXXFLAGS"]:
     env_metal["CXXFLAGS"].remove("-std=gnu++17")
-env_metal.Append(CXXFLAGS=["-std=c++20"])
+env_metal.Append(CXXFLAGS=["-std=gnu++20"])
 
 # Enable module support
 env_metal.Append(CCFLAGS=["-fmodules", "-fcxx-modules"])