ci: Migrate to macos-15-intel

Author: jorisv

.github/workflows/macos-linux-pixi.yml

@@ -38,7 +38,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, macos-13]
+        os: [ubuntu-latest, macos-latest, macos-15-intel]
         environment: [all, all-py310]
         build_type: [Release, Debug]