ci: make the build dir accessible when running w/o privileges

ci: make the build dir accessible when running w/o privileges

Otherwise the unprivileged part of test-execute gets silently skipped:

/* test_run_tests_unprivileged */
Successfully forked off '(test-execute-unprivileged)' as PID 20998.
...
pin_callout_binary: build dir binary: /home/runner/work/systemd/systemd/build/systemd-executor
pin_callout_binary: open(/home/runner/work/systemd/systemd/build/systemd-executor)=-13
Failed to pin executor binary: No such file or directory
(test-execute-unprivileged): manager_new, skipping tests: No such file or directory
(test-execute-unprivileged) succeeded.