_drop_privs_userfetch: ensure cwd is accessible

The cwd needs to be accessible in order for the multiprocessing
spawn start method to succeed:

  File "/usr/lib/python3.14/multiprocessing/spawn.py", line 235, in prepare
    os.chdir(data['dir'])
    ~~~~~~~~^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: '/root'

Bug: https://bugs.gentoo.org/965138
Signed-off-by: Zac Medico <[email protected]>

Author: zmedico

lib/portage/package/ebuild/fetch.py

@@ -120,6 +120,10 @@ def _drop_privs_userfetch(settings):
     os.setuid(int(spawn_kwargs["uid"]))
     os.umask(spawn_kwargs["umask"])
     portage.data.secpass = 1
+    try:
+        os.chdir(settings.get("HOME", "/"))
+    except Exception:
+        os.chdir("/")
 
 
 def _spawn_fetch(settings, args, **kwargs):