screen_file_contents_and_write occasionally failed with NotFound when
run in parallel with screen_temp_file_keeps_non_utf8_path_bytes: that
test sets TMPDIR to a short-lived directory via temp_env, and
std::env::temp_dir() evaluated outside temp_env's lock could resolve
into it. The temp-file section now runs under temp_env's lock with
TMPDIR unset. 25 consecutive runs of the core lib tests pass.