feat: use blacksmith to run vm tests instead of aarch64-linux self hosted

[samrose]

github/actions/nix-install-ephemeral/action.yml — Added extra-system-features = kvm and KVM permissions step

15 test files, 28 replacements — server.succeed( → server.execute( for all switch-to-configuration calls

1. Removed aarch64-linux from self-hosted runner map

The old config had aarch64-linux in the self-hosted section, pointing to our bare-metal runners. Since we're moving to Blacksmith,
there's no self-hosted aarch64-linux runner anymore. Without removing this, KVM packages would be assigned to a runner that doesn't
exist.

2. Changed KVM routing logic in get_runner_for_package

The old code sent ALL KVM packages to self-hosted runners (BUILD_RUNNER_MAP["self-hosted"].get(system)). With no self-hosted
aarch64-linux entry, this returns None and raises a ValueError. The new code checks: if Darwin, use self-hosted (macOS still needs
bare metal for KVM); if Linux, use ephemeral Blacksmith runners (which do have KVM via nested virtualization).

3. Removed max_memory_size from build_nix_eval_command and main()

This was the --max-memory-size flag passed to nix-eval-jobs to cap memory per eval worker at 3GB. TIt's unrelated to the KVM routing but was part of the same set of changes the flag was removed because
nix-eval-jobs handles memory management adequately on its own with the eval runner.

4. print → debug in run_nix_eval_jobs

Minor logging cleanup — changed a print to stderr into a debug() call so it only shows in debug mode instead of always polluting CI
output.