Fix machine run ENOEXEC for amd64 machines on arm64 host

[iamvirul]

Type of Change

• Bug fix

Motivation and Context

Fixes #1843.

MachinesService.boot() calls ClientKernel.getDefaultKernel(for: .current) when no custom kernel path is configured. On Apple Silicon, .current resolves to .linuxArm, so AMD64 machines boot with the default-arm64 kernel binary instead of default-amd64.

The two are distinct binaries. The default-amd64 kernel is compiled with the Rosetta and binfmt_misc support required to run AMD64 userspace on ARM64 hosts. Without it, non-root exec processes fail with ENOEXEC (errno=8) when attempting to run AMD64 binaries through the binfmt_misc handler.

This is why the machine itself boots (PID 1 runs as root, unaffected) but container machine run -n ubuntu uname fails (uses the host user uid, which is non-root).

The custom kernel path branch in the same function already handled this correctly via self.systemPlatform(from: state.snapshot.configuration.platform). The default kernel path branch was inconsistent with it.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Reproduced with the steps in #1843 and confirmed container machine run -n ubuntu uname -a succeeds after the fix.