Switch nightly builds to macOS Tahoe runner (#2231)

Use warp-macos-26-arm64-6x for nightly builds to get the latest Xcode
toolchain. Deployment target stays at 14.0 (Sonoma), so built binaries
remain compatible with Sonoma, Sequoia, and Tahoe.

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

.github/workflows/nightly.yml

@@ -100,7 +100,7 @@ jobs:
   build-sign-notarize-nightly:
     needs: decide
     if: needs.decide.outputs.should_build == 'true'
-    runs-on: warp-macos-15-arm64-6x
+    runs-on: warp-macos-26-arm64-6x
     timeout-minutes: 20
     steps:
       - name: Checkout build ref