[CI] Add support for basic testing of ARM if-branch (e.g. brew fetch)

[cho-m]

Provide a detailed description of the proposed feature

This is to improve checks on homebrew-cask CI by adding some support for basic (non-install) tests for ARM branch.

For example, at minimum doing something like a brew fetch to check valid URL/SHA. Perhaps an equivalent of brew audit --online test for livecheck.

This may either require a flag in brew to "fake" Hardware for certain commands, or a CI-specific test logic rather than directly running brew commands.

Potentially consider scoping out if possible to run simple tests for other macOS version branches too (e.g. Mojave and older, or Monterey beta, or ...). This would allow us to detect dead branches better.

What is the motivation for the feature?

Since homebrew-cask don't have an ARM CI node, the CI doesn't seem to test the ARM branch.

Given that ARM-specific download URLs are becoming common, this means Homebrew maintainers may need to manually confirm ARM branch.

Having a test in CI would help check correct url/sha/etc and reduce manual confirmation overhead.

Example use case

I have seen some PRs that showed CI passing, but when I locally checked out PR, the brew fetch failed.

For example, in #107488 (corresponding commit to change ARM url 696bb78)

• Run with incorrect ARM url https://github.com/Homebrew/homebrew-cask/actions/runs/958546506
• Run with correct ARM url https://github.com/Homebrew/homebrew-cask/actions/runs/958888936

[SMillerDev]

I wonder if, instead of faking the OS, it wouldn't be better for casks to implement a DSL similar to a formula bottle block to define what a specific url supports.

[yurikoles]

I like ArchLinux' PKGBUILD approach, which allows defining architecture-specific sources as source_x86_64, and corresponding checksum as sha256sums_x86_64. We may apply this approach to HBC DSL as url_x86_64 / sha256_x86_64 and url_aarch64 / sha256_aarch64.

[bevanjkay]

This was completed by #143743