build.yml: Use nix-fast-build to build on remote

[henrirosten]

Description of changes

Start using nix-fast-build in the build.yml workflow to execute all build operations completely on the remote end.

Removes the step 'Maximize space available on rootfs' since with nix-fast-build, we no longer download the full build closure on the github-hosted runner. Furthermore, with the --no-download option, we also skip the download of the final build result (ghaf image), so we no longer need to free up the storage on the github runner.

This change also removes the upload to https://ghaf-dev.cachix.org from the build workflow: it is no longer needed, since builds are already cached on the remote builders' local nix store. Jenkins builds will take care of uploading the build binaries to a separate nix binary cache, maintained with ghaf-infra.

With the above summarized changes, build times on the build.yml workflow will significantly reduce. It should also completely remove the hash mismatch issues on the build workflow as builds and also evaluation operations are executed on the remote.

This change was tested on a fork, as an example, see: https://github.com/henrirosten/ghaf/actions/runs/11587433044.

Checklist for things done

• [x] Summary of the proposed changes in the PR description
• [ ] More detailed description in the commit message(s)
• [x] Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• [x] Contribution guidelines followed
• [ ] Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• [ ] PR linked to architecture documentation and requirement(s) (ticket id)
• [x] 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
• [x] Author has run make-checks and it passes
• [x] All automatic Github Action checks pass - see actions
• [x] 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?