Select Xcode 26.6 for CI on the macOS 26 runner image (#2726)

GitHub rolled the macos-latest alias from macos-15-arm64 to macos-26-arm64
between June 3 and July 22. Xcode 26.2 still exists on the new image, but
its macOS-host xctest runner crashes at session bootstrap on a macOS 26
host ("Early unexpected exit, operation never finished bootstrapping"),
which is why only the Unix leg went red: it is the only leg running
native macOS test bundles. The simulator and build-only legs are
unaffected.

Xcode 26.6 is the new image's default toolchain and runs the full suite
green on a macOS 26 host locally.

Author: freak4pc

.github/workflows/tests.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Select Xcode 26
-        run: sudo xcode-select -s /Applications/Xcode_26.2.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
       - name: Run Tests
         run: CI=1 ./scripts/all-tests.sh "${{ matrix.environment }}"