Only set extra_groups for mkdir() if we were invoked as root

The extra groups of the user are not mapped into the user namespace
we create and as such can't be set when invoking a subprocess if
we weren't invoked as root.

mkosi/user.py

@@ -76,7 +76,7 @@ def mkdir(cls, path: Path) -> Path:
             ["mkdir", "--parents", path],
             user=cls.uid if cond else os.getuid(),
             group=cls.gid if cond else os.getgid(),
-            extra_groups=cls.extra_groups() if cond else None,
+            extra_groups=cls.extra_groups() if cls.invoked_as_root and cond else None,
         )
         return path