[CI] GitHub Actions - Support macOS testing

[shahmishal]

No description provided.

[owenv]

Failures:
incrementalMetalLinkWithCodeSign, singleFileCompileMetal - failing because the Metal toolchain is not installed
cachedBuild - is seeing some unexpected cache misses

@shahmishal for the time being, I think we should probably only bring up the Xcode 26b6 job since we've only been maintaining compatibility with 16.2 for use in Swift CI

[jakepetroules]

Failures: incrementalMetalLinkWithCodeSign, singleFileCompileMetal - failing because the Metal toolchain is not installed cachedBuild - is seeing some unexpected cache misses

@shahmishal for the time being, I think we should probably only bring up the Xcode 26b6 job since we've only been maintaining compatibility with 16.2 for use in Swift CI

Note that there's three: 16.2, 16.3, and 26. And 16.3 is completely passing.

Are you suggesting we bring up only 26, or only 26 + 16.3?

[jakepetroules]

We can also just merge #773 and then look at adding 26 later.

[shahmishal]

@swift-ci test