mat2: Fix the --no-sandbox argument

The --no-sandbox argument was parsed incorrectly, meaning no sandbox was
used when it was absent and the sandbox being used when it was present.

mat2

@@ -57,8 +57,8 @@ def create_arg_parser() -> argparse.ArgumentParser:
                         ', '.join(p.value for p in UnknownMemberPolicy))
     parser.add_argument('--inplace', action='store_true',
                         help='clean in place, without backup')
-    parser.add_argument('--no-sandbox', dest='sandbox', action='store_true',
-                        default=False, help='Disable bubblewrap\'s sandboxing')
+    parser.add_argument('--no-sandbox', dest='sandbox', action='store_false',
+                        default=True, help='Disable bubblewrap\'s sandboxing')
 
     excl_group = parser.add_mutually_exclusive_group()
     excl_group.add_argument('files', nargs='*', help='the files to process',