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

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

Author: rsmmr

.cirrus.yml

@@ -4,6 +4,10 @@ environment:
     # Enforce sequential JIT'ing of files for controlled memory usage.
     HILTI_JIT_SEQUENTIAL: 1
 
+    # Images for macOS
+    IMAGE_MACOS_BIG_SUR: big-sur-xcode
+    IMAGE_MACOS_CATALINA: catalina-xcode
+
 clang9_zeek_ubuntu_debug_task:
   container:
     dockerfile: ci/Dockerfile
@@ -211,7 +215,7 @@ clang11_zeek_nightly_task:
 
 macos_catalina_task:
   osx_instance:
-    image: catalina-base
+    image: $IMAGE_MACOS_CATALINA
 
   always:
     ccache_cache:
@@ -248,7 +252,7 @@ macos_catalina_task:
 
 macos_big_sur_task:
   osx_instance:
-    image: big-sur-base
+    image: $IMAGE_MACOS_BIG_SUR
 
   always:
     ccache_cache:
@@ -371,7 +375,7 @@ homebrew_catalina_task:
   skip: $CIRRUS_BRANCH != 'main' && $CIRRUS_TAG == ''
 
   osx_instance:
-    image: catalina-base
+    image: $IMAGE_MACOS_CATALINA
 
   script:
   - brew tap zeek/zeek
@@ -382,7 +386,7 @@ homebrew_big_sur_task:
   skip: $CIRRUS_BRANCH != 'main' && $CIRRUS_TAG == ''
 
   osx_instance:
-    image: big-sur-base
+    image: $IMAGE_MACOS_BIG_SUR
 
   script:
   - brew tap zeek/zeek