Enable ubsan stack trace printing on community job

Try to do this using "variables" -- apparently these get uppercased
and are made available as environment variables.

Author: nikic

azure/community_job.yml

@@ -17,6 +17,8 @@ jobs:
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
       vmImage: 'ubuntu-latest'
+    variables:
+      ubsan_options: 'print_stacktrace=1'
     steps:
     - template: apt.yml
     - script: |