CI: Update macOS Intel target version in workflow

GitHub is deprecating macOS 13 runners -- the only remaining Intel runner is now macOS 15, and it is scheduled for retirement in 2027, at which point we will no longer be able to provide Intel weeklies.

.github/workflows/sub_weeklyBuild.yml

@@ -63,7 +63,7 @@ jobs:
         include:
           - { target: linux-64, os: ubuntu-22.04 }
           - { target: linux-arm64, os: ubuntu-22.04-arm }
-          - { target: osx-64, os: macos-13 }
+          - { target: osx-64, os: macos-15-intel }
           - { target: osx-arm64, os: macos-latest }
           - { target: win-64, os: windows-latest }
       fail-fast: false