chore: use pinned images for Yamato testing (#438)

Author: sindharta

.yamato/StreamingImageSequence.metafile

@@ -9,9 +9,9 @@ test_editors:
 test_platforms:
   - name: win
     type: Unity::VM
-    image: package-ci/win10:stable
+    image: package-ci/win10:v1.20.1
     flavor: b1.medium
   - name: mac
     type: Unity::VM::osx
-    image: package-ci/mac:stable       
+    image: package-ci/mac:v1.20.0-1079282
     flavor: m1.mac