🔨 Fix LPC176x debug build

See MarlinFirmware#23635

buildroot/share/PlatformIO/scripts/common-cxxflags.py

@@ -29,7 +29,7 @@ def add_cpu_freq():
 	# It useful to keep two live versions: a debug version for debugging and another for
 	# release, for flashing when upload is not done automatically by jlink/stlink.
 	# Without this, PIO needs to recompile everything twice for any small change.
-	if env.GetBuildType() == "debug" and env.get('UPLOAD_PROTOCOL') not in ['jlink', 'stlink']:
+	if env.GetBuildType() == "debug" and env.get('UPLOAD_PROTOCOL') not in ['jlink', 'stlink', 'custom']:
 		env['BUILD_DIR'] = '$PROJECT_BUILD_DIR/$PIOENV/debug'
 
 	# On some platform, F_CPU is a runtime variable. Since it's used to convert from ns