Github action parallel nix eval

[henrirosten]

Description of changes

Re-introduce the following changes from #947:

• Move the nix evaluation step to its own workflow eval.yml, which runs the nix evaluation in parallel, splitting the evaluation to 8 subsets, each of which are executed on a separate github-hosted runner. In testing, this improves the eval action runtime from ~35 minutes down to ~7 minutes.
• eval.sh allows filtering the evaluated targets via the -t option: the initial version in this PR will evaluate all Ghaf flake outputs that match the default filter 'packages.*'.

Apparently Ghaf flake does not evaluate with nix flake show --all-systems the way suggested in #947 (comment), which might hint other evaluation problems in Ghaf flake.

For now, we need this check to keep the Ghaf flake clear from any (new) evaluation errors.

Checklist for things done

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• PR linked to architecture documentation and requirement(s) (ticket id)
• Test procedure described (or includes tests). Select one or more:
  • Tested on Lenovo X1 x86_64
  • Tested on Jetson Orin NX or AGX aarch64
  • Tested on Polarfire riscv64
• Author has run make-checks and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status
• Change requires full re-installation
• Change can be updated with nixos-rebuild ... switch

Instructions for Testing

• List all targets that this applies to:
• Is this a new feature
  • List the test steps to verify:
• If it is an improvement how does it impact existing functionality?