Speed up the new CI jobs introduced in #7653

[Mikolaj]

The idea is to upload artifacts in the same workflow and use them in the dogfooding workflow instead of building anew each time.

That should probably save around 20% of time spent in that whole workflow or a couple percent for the whole CI run (more once we manage to run Windows with more GHCs).

There's TODO in the CI template in that #7653 PR. The GitHub Actions docs are at https://docs.github.com/en/actions/guides/storing-workflow-data-as-artifacts

[jneira]

Another option could be cache effectively the build dir (now the workflow is attempting to do it but validate.sh uses a custom build dir instead dist-newstyle)

[andreabedini]

Looping myself in as I just spent some times with build artefacts on a different project.