Switch the Cirrus macOS images to *-xcode-12.x.

This works around cirruslabs/cirrus-ci-docs#878.

Author: rsmmr

.cirrus.yml

@@ -211,7 +211,7 @@ clang11_zeek_nightly_task:
 
 macos_catalina_task:
   osx_instance:
-    image: catalina-base
+    image: catalina-xcode-12.2
 
   always:
     ccache_cache:
@@ -248,7 +248,7 @@ macos_catalina_task:
 
 macos_big_sur_task:
   osx_instance:
-    image: big-sur-base
+    image: big-sur-xcode-12.5
 
   always:
     ccache_cache: