ci: add aarch64-linux support for checks, builds, and releases

[codgician]

Summary

Add aarch64-linux support to the CI/CD pipeline, enabling ARM64 Windows releases, fixes #534

Changes

• run_checks.yml: Run flake checks on both x86_64-linux and aarch64-linux using matrix strategy
• run_build.yml: Build tarballs for both architectures using matrix strategy
  • nixos.wsl (x86_64) on ubuntu-24.04
  • nixos.aarch64.wsl (aarch64) on ubuntu-24.04-arm
• run_validate.yml: Validate both tarballs using matrix strategy
• run_tests.yml: Use tarball-x86_64 artifact name (tests remain x86_64-only as windows-11-arm runner lacks nested virtualization support required for WSL2)
• on_release.yml: Upload both tarballs with checksums on release
• flake.nix: Add aarch64 nixosConfiguration using a shared mkSystem helper

Release Artifacts

With this change, releases will include:

• nixos.wsl + nixos.wsl.sha256 (x86_64)
• nixos.aarch64.wsl + nixos.aarch64.wsl.sha256 (aarch64)

Testing

Tested on Surface Pro 11 (Snapdragon X Elite)

[nzbr]

Oh so there finally are ARM runners! Thanks for adding support