change VM image for CI

Author: hiroki-o

.yamato/upm-ci-AssetGraph.yml

@@ -10,7 +10,7 @@ platforms:
     flavor: b1.large
   - name: mac
     type: Unity::VM::osx
-    image: buildfarm/mac:stable
+    image: package-ci/mac:stable
     flavor: m1.mac
 ---
 pack: