Move all workflow builds to remote builders

Description of changes

Stop building any ghaf targets directly on github-hosted runners in the build.yml workflow. Instead, delegate all builds to remote builders as configured in github action variable BUILDER_MACHINE_CONFIG. Currently, all builds will be delegated to hetzarm.vedenemo.dev (aarch64) and build4.vedenemo.dev (x86_64). We hope this change also makes it possible to start building ghaf's biggest targets (package-lenovo-x1-carbon-gen11-debug and package-generic-x86_64-debug) in the build.yml github action. Adding those targets to the build.yml will be done later in a follow-up PR.
Make builds in build.yml start using binary caches as configured in the ghaf flake by setting the accept-flake-config = true and by making the github action user 'runner' a nix trusted user on the github hosted runner. This way, we don't have to change the build.yml every time we change the binary cache config in ghaf flake, but the github action builds will automatically follow the ghaf flake configuration.
Update cachix/install-nix-action on all workflows to its current latest version.

This change was tested in a fork at: https://github.com/henrirosten/ghaf.

[x] Summary of the proposed changes in the PR description
[x] 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