tools: Support FLUTTER_HOST_ARCH in update_dart_sdk scripts

[cbracken]

Adds support for overriding the host CPU architecture via a FLUTTER_HOST_ARCH environment variable when pre-caching binaries. Updates update_dart_sdk.sh and update_dart_sdk.ps1, to make use of this to pull down the specified Dart SDK.

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

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

This is pre-factoring prior to updating the tool's precache code.

No test changes since by default, this behaves exactly as today and this is "tested" by the build itself on CI. The followup that updates the tool's precache code will exercise this and add tests for it.

Issue: #189144

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[gemini-code-assist]

Code Review

This pull request introduces the FLUTTER_HOST_ARCH environment variable to override host architecture detection in both the Windows PowerShell and Unix shell scripts for updating the Dart SDK. The review feedback suggests normalizing the architecture string to lowercase and mapping alternative architecture names (such as aarch64) to ensure robust and consistent architecture matching across different environments.

[cbracken]

Requested a test exemption at https://discord.com/channels/608014603317936148/1290464157765865552/1533648794619613184

[stuartmorgan-g]

test-exempt: groundwork for follow-up containing tests.

[jtmcdole]