bump the number of executors on the jenkins build docker image

This image isn't configured to spawn extra workers, so just bump the
number of executors on the master instance so that certain jobs (which
trigger downstream jobs) will work.

docker/build/jenkins_build/ansible_overrides.yml

@@ -45,3 +45,8 @@ jenkins_common_main_labels:
   - 'dsl-seed-runner'
   - 'backup-runner'
   - 'jenkins-worker'  # added this
+
+# We're running all our jobs on the Jenkins Master by default (one container
+# only), so we need to bump up the number of executors for some jobs with
+# downstream jobs to work correctly.
+jenkins_common_main_num_executors: 6