ci: fix after_script for out-of-tree tests

The commands in after_script run in a new shell, separate from
before_script and script commands. Since the pytest.log.txt is for out
of tree tests is present in the build directory, switch to it (if set)
before running any postprocessing commands.

(cherry picked from commit 86043b0bbe)

.gitlab-ci.yml

@@ -408,6 +408,7 @@ cross-version-config-tests:
       "$PYTEST" --junit-xml="$CI_PROJECT_DIR"/junit.xml -n "$TEST_PARALLEL_JOBS" | tee pytest.out.txt
     - '( ! grep -F "grep: warning:" pytest.out.txt )'
   after_script:
+    - test -n "${OUT_OF_TREE_WORKSPACE}" && cd "${OUT_OF_TREE_WORKSPACE}"
     - *display_pytest_failures
 
 .system_test_legacy: &system_test_legacy