Switch to a stable tag on our bokken image. (#2799)

.yamato/csharp-tests.yml

 
 agent:
   type: Unity::VM::osx
-  image: ml-agents/ml-agents-bokken-mac:latest
+  image: ml-agents/ml-agents-bokken-mac:stable
   flavor: i1.small
 
 variables:

.yamato/standalone-build-test.yml

 
 agent:
   type: Unity::VM::osx
-  image: ml-agents/ml-agents-bokken-mac:latest
+  image: ml-agents/ml-agents-bokken-mac:stable
   flavor: i1.small
 
 variables: