site_cache_dir: Use /var/cache again instead of /var/tmp on UNIX

This directory was changed from /var/cache to /var/tmp in tox-dev#148 due to
permissions issues. However, /var/tmp is an insecure location to store
anything with a predictable filename, because any other user could
have written it first. This leads to vulnerabilities categorized under
CWE-377 and CAPEC-149.

To deal with the permissions issues, applications should put their own
cache data in a subdirectory of /var/cache (e.g. /var/cache/cups), and
the application’s package is responsible for ensuring the subdirectory
exists and giving it the correct permissions.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/platformdirs/unix.py

@@ -99,8 +99,8 @@ def user_cache_dir(self) -> str:
 
     @property
     def site_cache_dir(self) -> str:
-        """:return: cache directory shared by users, e.g. ``/var/tmp/$appname/$version``"""
-        return self._append_app_name_and_version("/var/tmp")  # noqa: S108
+        """:return: cache directory shared by users, e.g. ``/var/cache/$appname/$version``"""
+        return self._append_app_name_and_version("/var/cache")
 
     @property
     def user_state_dir(self) -> str: