build.yml: Build x86 generic and x1 targets

[henrirosten]

Description of changes

Start building ghaf targets generic-x86_64-debug and lenovo-x1-carbon-gen11-debug in the build.yml workflow.
This should work after #865 since all the builds are now done on the remote builders, not on the github-hosted runners. Earlier attempts to enable generic-x86_64-debug or lenovo-x1-carbon-gen11-debug on the build workflow have failed due to two reasons: (1) github-hosted runners not having enough resources to build these massive targets and (2) hash mismatches I suspect have occurred mostly due to github-hosted runners running the builds on Ubuntu-based VMs. After #865, both those issues should be resolved by running all the builds on the remote (nix) builders.

Depends-on: #865: Do not merge before #865 is merged.

This change was tested in a fork at: https://github.com/henrirosten/ghaf. Especially, see the generic-x86_64-debug and lenovo-x1-carbon-gen11-debug builds on the following test PRs:

• Update nixos-hardware henrirosten/ghaf#68
• Test disko update henrirosten/ghaf#69
• Test microvm update henrirosten/ghaf#70
• Update all inputs henrirosten/ghaf#71

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?