This updates our build script and CI to support Xcode 26 and macOS
Tahoe. **This doesn't update the Ghostty app to resolve any Tahoe
issues.**
For CI, we've added a new build job that runs on macOS Tahoe with Xcode
26. I've stopped short of updating our tip release job, since I think I
want to wait until I verify a bit more about Tahoe before we flip that
bit. Also, ideally, we'd run Xcode 26 on Sequoia (macOS 15) for
stability reasons and Namespace doesn't have Xcode 26 on 15 yet.
For builds, this updates our build script to find Metal binaries using
`xcodebuild -find-executable` instead of `xcrun`. The latter doesn't
work with Xcode 26, but the former does and also still works with older
Xcodes. I'm not sure if this is a bug but I did report it: FB17874042.
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