t0024, t5000: clear variable UNZIP, use GIT_UNZIP instead

t0024, t5000: clear variable UNZIP, use GIT_UNZIP instead

InfoZIP's unzip takes default parameters from the environment variable
UNZIP.  Unset it in the test library and use GIT_UNZIP for specifying
alternate versions of the unzip command instead.

t0024 wasn't even using variable for the actual extraction.  t5000
was, but when setting it to InfoZIP's unzip it would try to extract
from itself (because it treats the contents of $UNZIP as parameters),
which failed of course.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>