Add zipped jobs.yaml to build-prow-images PR (#744)

Issue #, if available:

Description of changes:
- Include prow/jobs/jobs.yaml.gz as source file in PR generated by the build-prow-images script.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Author: knottnt

prow/jobs/tools/cmd/build-prow-images.sh

@@ -72,7 +72,7 @@ fi
 if [ -n "$BUILT_JOB_TAGS" ] || [ -n "$BUILT_AGENT_WORKFLOW_TAGS" ] || [ -n "$BUILT_PLUGIN_TAGS" ]; then
   if [ -n "$BUILT_JOB_TAGS" ]; then
     PR_DESCRIPTION+="Built and pushed prow job images:"$'\n'"$BUILT_JOB_TAGS"$'\n\n'
-    SOURCE_FILES+="prow/jobs/jobs.yaml:prow/jobs/jobs.yaml"
+    SOURCE_FILES+="prow/jobs/jobs.yaml:prow/jobs/jobs.yaml,prow/jobs/jobs.yaml.gz:prow/jobs/jobs.yaml.gz"
   fi
 
   if [ -n "$BUILT_AGENT_WORKFLOW_TAGS" ]; then