ci: allow github-actions print test failures again

eab5dba (ci: wire up Meson builds, 2024-12-13) added two instances
of a very similar construct

    FAILED_TEST_ARTIFACTS=${TEST_OUTPUT_DIRECTORY:-t}/failed-test-artifacts

one to ci/lib.sh and the other to ci/print-test-failures.sh
Unfortunately, the latter had a typo causing shell to emit "Bad
substitution".  Fix it.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

ci/print-test-failures.sh

@@ -41,7 +41,7 @@ do
 		case "$CI_TYPE" in
 		github-actions)
 			mkdir -p failed-test-artifacts
-			echo "FAILED_TEST_ARTIFACTS=${TEST_OUTPUT_DIRECTORY:t}/failed-test-artifacts" >>$GITHUB_ENV
+			echo "FAILED_TEST_ARTIFACTS=${TEST_OUTPUT_DIRECTORY:-t}/failed-test-artifacts" >>$GITHUB_ENV
 			cp "${TEST_EXIT%.exit}.out" failed-test-artifacts/
 			tar czf failed-test-artifacts/"$test_name".trash.tar.gz "$trash_dir"
 			continue