python-3.8 changed the default tar format to PAX_FORMAT. This caused
issues in our testsuite with package extraction of files with UTF-8
characters as we run the tests under the C locale.

sycn600.py:
error: error while reading package /tmp/pactest-xuhri4xa/var/cache/pacman/pkg/unicodechars-2.0-1.pkg.tar.gz: Pathname can't be converted from UTF-8 to current locale.

Set format back to GNU_FORMAT.

Signed-off-by: Allan McRae <allan@archlinux.org>