Fix GitHub Actions: install Metal Toolchain for Xcode 26+

Author: schwa

.github/workflows/swift.yml

@@ -11,6 +11,8 @@ jobs:
       - uses: maxim-lobanov/setup-xcode@v1
         with:
           xcode-version: ${{ env.XCODE_VERSION }}
+      - name: Install Metal Toolchain
+        run: xcodebuild -downloadComponent MetalToolchain
       - uses: actions/checkout@v3
       - run: swift build -v
       - run: swift test -v