Use discovery script to find all tarballs and use slow-validate flavour for build-master

These patches do two things.

1. Use ./discover-tarball script for build-9.4 and build-master jobs to find the latest tarball which is suitable on these branches. This fixes #61 (closed) by looking for a bindist from the branch rather than getting confused by release and nightly pipelines.
2. Use the deb10-numa-slow-validate job for validate runs, this compiler has runtime assertions enabled and already caught one assertion failure.