CI: switch to supported macOS VM image on Cirrus CI

Apparently the image previously in use has been retired and the jobs
were automatically upgraded to this image. Switch it to avoid possible
future breakage.

Author: dnicolodi

.cirrus.yml

@@ -84,6 +84,6 @@ freebsd_task:
 
 macos-arm64_task:
   macos_instance:
-    image: ghcr.io/cirruslabs/macos-sonoma-xcode
+    image: ghcr.io/cirruslabs/macos-runner:sonoma
   install_script: brew install git ninja python@3.11
   << : *test