Use local directory in CI

[vkuncak]

In some cases sbt fails due to a lock held on the directory it wishes to use. Currently it seems mostly happening on laraquad01:

https://github.com/epfl-lara/stainless/actions/runs/10874017475/job/30170826406

Perhaps this would be fixed by specifying an explicit temporary directory within the stainless checkout, like we do to download solvers, instead of relying on global /var paths. Perhaps some of these options in the sbt bach script will help (but we need to also ensure we call a new enough sbt script):

  --sbt-dir   <path>  path to global settings/plugins directory (default: ~/.sbt)
  --sbt-boot  <path>  path to shared boot directory (default: ~/.sbt/boot in 0.11 series)
  --sbt-cache <path>  path to global cache directory (default: operating system specific)
  --ivy       <path>  path to local Ivy repository (default: ~/.ivy2)