Try yet different image

Author: aurimasc

.yamato/upm-ci.yml

@@ -124,7 +124,7 @@ test_iOS_{{ editor.version }}:
   name: Test {{ editor.version }} on iOS
   agent:
     type: Unity::mobile::iPhone
-    image: mobile/ios-macos-11:latest
+    image: slough-ops/macos-10.15-base:stable
     flavor: i1.large
   commands:
     - brew tap --force-auto-update unity/unity [email protected]:unity/homebrew-unity.git