gettext tests: export the restored GIT_TEST_GETTEXT_POISON

gettext tests: export the restored GIT_TEST_GETTEXT_POISON

6cdccfce ("i18n: make GETTEXT_POISON a runtime option", 2018-11-08)
made the gettext-poison test a runtime option (which was a good
move) and adjusted the test framework so that Git commands we run as
part of the framework, as opposed to the ones that are part of the
test proper, are not affected by the setting.  The original value
for the GIT_TEST_GETTEXT_POISON environment variable is saved away
in another variable and gets unset, and then later the saved value
is restored to the environment variable.

But the code forgot to export the variable again, which is necessary
to restore the "export" bit that was lost when the variable was unset.

Signed-off-by: Junio C Hamano <gitster@pobox.com>