Try cache mkdir in XDG handling

Author: gpotter2

scapy/main.py

@@ -73,7 +73,12 @@ def _probe_xdg_folder(var, default, *cf):
         # https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
         # "If, when attempting to write a file, the destination directory is
         # non-existent an attempt should be made to create it with permission 0700."
-        path.mkdir(mode=0o700, exist_ok=True)
+        try:
+            path.mkdir(mode=0o700, exist_ok=True)
+        except Exception:
+            # There is a gazillion ways this can fail. Most notably,
+            # a read-only fs.
+            return None
     return path.joinpath(*cf).resolve()