"docker-privileged-xl" in build runners for Allen

Specify tag docker-privileged-xl in build runners (as suggested here: https://cern.service-now.com/service-portal/?id=kb_article&n=KB0005851). Change to initially only run one build, before the other start.

First build only for hlt1_pp_default and CPU. If that succeeds, also launch CUDA and HIP builds, as well as hlt1_pp_validation in the minimal build matrix.

Reduce the full pipeline to jobs with scifi v6 samples, and a build with tests, contracts and ROOT enabled,

Partially solves #277 (closed)