.gitlab-ci/buildgrid-compose.yml: Switch to buildbox-worker
Use buildbox-worker with buildbox-run-bubblewrap and buildbox-fuse from the buildbox-nightly docker image instead of the Python buildbox worker as the latter still uses the obsolete buildbox
command.
buildbox-worker launch is delayed as it fails if started before the BuildGrid server is ready.
This change is cherry-picked from !1816 (merged) as it's also needed for !1818 (merged).