`macos-latest-xlarge` now resolves to macOS 26, which ships only Xcode 26
(Swift 6.2) and no longer includes Xcode 16. The Swift-analysing PR checks
select Xcode 16 because CodeQL CLI versions before 2.24.0 only support Swift
up to 6.1, so `xcode-select -s /Applications/Xcode_16.app` fails on macOS 26.
Pin these checks to macOS 15, where Xcode 16 is still available, so we keep
testing the full matrix of supported CodeQL versions. This also pre-empts the
`swift-custom-build` check failing once plain `macos`/`macos-latest` migrates.
See https://github.com/actions/runner-images/issues/14167.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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