tools: Support --host-arch option in flutter precache

[cbracken]

Adds a --host-arch=<x64|arm64> option to flutter precache and adds support for the FLUTTER_HOST_ARCH environment variable in OperatingSystemUtils.hostPlatform.

When specified, this overrides the default hardware detection check (sysctl hw.optional.arm64 on macOS), and causes flutter precache to download host engine artifacts for the specified architecture instead.

This is required to allow arm64 macOS CI hosts to download and cache x64 host engine artifacts when cross-packaging x64 Flutter SDK release archives on an arm64 host in the packaging/packaging recipe in packaging.py.

See: https://flutter.googlesource.com/recipes/+/refs/heads/main/recipes/packaging/packaging.py

Issue: #189144

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[gemini-code-assist]

Code Review

This pull request adds a --host-arch option to the precache command and introduces a FLUTTER_HOST_ARCH environment variable override to allow overriding the host platform architecture. The review feedback suggests refactoring _MacOSUtils.hostPlatform to delegate to super.hostPlatform to avoid code duplication, and simplifying the PrecacheCommand constructor and associated tests by removing the redundant os parameter and field.

[bkonyi]

LGTM overall, just a couple of nits.

[bkonyi]

LGTM w/ one question

[bkonyi]

Can we not just make _osUtils public?

[jtmcdole]

A+, no notes. Just an informative "if you ever want to use multiple keys from that map", this pattern works:

class Platform {
  final environment = <String, Object?>{};
}

void main() async {
  final _platform = Platform()
    ..environment['FLUTTER_HOST_ARCH'] = 'arch128'
    ..environment['FOO'] = 'bar';

  if (_platform.environment case {
    'FLUTTER_HOST_ARCH': final String overrideArch,
    'FOO': final String foo,
  }) {
    print((overrideArch: overrideArch, foo: foo));
    // (foo: bar, overrideArch: arch128)
 }
}