Imitating cac42e47 (ci: avoid using the deprecated `set-env`
construct, 2020-11-07), avoid deprecated ::set-env and use the
recommended alternative instead in print-test-failures.sh
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>