changed to stable linux image

.yamato/environments.yml

     standalone-platform: StandaloneOSX
   - name: ubuntu
     type: Unity::VM
-    image: package-ci/ubuntu:latest
+    image: package-ci/ubuntu:stable
     flavor: b1.large
 
 performance_platforms:
     standalone-platform: StandaloneOSX
   - name: ubuntu
     type: Unity::VM
-    image: package-ci/ubuntu:latest
+    image: package-ci/ubuntu:stable
     flavor: b1.large
 
 performance_suites: