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>