git_clan_flake_toplevel: gracefully handle permission errors

since this code is used as a default in the cli parser, we should not
crash on OSError because we wouldn't be abe to perform basic tasks such
as --help.

pkgs/clan-cli/clan_lib/dirs/__init__.py

@@ -29,9 +29,11 @@ def get_clan_flake_toplevel_or_env() -> Path | None:
 
 
 def get_clan_flake_toplevel() -> Path | None:
-    return find_toplevel([".clan-flake", ".git", ".hg", ".svn", "flake.nix"])
-
-
+    try:
+        return find_toplevel([".clan-flake", ".git", ".hg", ".svn", "flake.nix"])
+    except OSError:
+        # We might run clan from a directory that is not readable.
+        return None
 
 
 def clan_key_safe(flake_url: str) -> str: