Probably this would be the fix:

Path defaultTempDir()
{
    auto dir = settings.tempDir.get().or_else([] {
        return getEnvNonEmpty("TMPDIR").and_then([](auto val) -> std::optional<Path> {
#if __APPLE__
            /* On macOS, don't use the per-session TMPDIR (as set e.g. by
               sshd). This breaks build users because they don't have access
               to the TMPDIR, in particular in ‘nix-store --serve’. */
            if (val.starts_with("/var/folders/")) {
                return std::nullopt;
            }
#endif
            return val;
        });
    }).value_or("/tmp");
    
    return canonPath(dir, true);
}