CI: Replace spot instances with dedicated runners for build jobs

This is part of %(OKR 2022Q3 - 4.3) - Faster CI.

#3613 (closed) showed that we could gain a few minutes by having the build jobs run on dedicated runners (or more specifically runners that we don't kill) that use dune-cache.

Important: this is only for build jobs.

Questions:

• how many dedicated runners do we want?
  • how many build jobs do we have in parallel around the day (max / average / median…)?
  • what is the cost?
• is it possible to fallback on spot instances if all dedicated runner spots are taken? (check GitLab runner if there is a priority system)