Update Darwin x86_64 build runner from macos-13 to macos-15-intel

macos-13 runners were fully retired by GitHub in December 2025,
causing the Darwin x86_64 build job to be instantly cancelled.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pskrbasu

.github/workflows/buildimage.yml

@@ -16,7 +16,7 @@ env:
 jobs:
   build-osx:
     name: Build for Darwin x86_64
-    runs-on: macos-13
+    runs-on: macos-15-intel
     steps:
       - name: Install PostgreSQL@14
         run: brew install --force postgresql@14