ci(nightly): run iOS build on macos-26 with Xcode selected

The nightly iOS job ran on macos-15 without selecting full Xcode, so
build-ios-ipa.sh bailed at its xcrun --sdk iphoneos --find metal check
(Command Line Tools have no iOS SDK). build-all's iOS job builds on
macos-26 after sudo xcode-select -switch and is green, so match that:
same runner + select /Applications/Xcode.app before the build.

.github/workflows/nightly.yml

@@ -238,9 +238,11 @@ jobs:
     needs: gate
     if: ${{ needs.gate.outputs.proceed == 'true' }}
     continue-on-error: true
-    runs-on: macos-15
+    runs-on: macos-26
     steps:
       - uses: actions/checkout@v7
+      - name: Select Xcode
+        run: sudo xcode-select -switch /Applications/Xcode.app
       - name: Build unsigned IPA
         run: ./platforms/ios/scripts/build-ios-ipa.sh