Downgrade Jenkins plugin postbuildscript to version 3.0.0 (sysadmin#17847)

This reverts part of e329f31e.

After upgrading the Jenkins postbuildscript plugin to the latest
available version (3.1.0-375.v3db_cd92485e1), the following error
started to happen when testing jobs:

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zen@jenkins ~> sudo -u jenkins /usr/local/sbin/deploy_jenkins_jobs test /tmp/tmp.HkMEI04rMa/jenkins-jobs
INFO:jenkins_jobs.cli.subcommand.update:Updating jobs in ['/tmp/tmp.HkMEI04rMa/jenkins-jobs', '/tmp/tmp.HkMEI04rMa/jenkins-jobs/Tails_ISOS', '/tmp/tmp.HkMEI04rMa/jenkins-jobs
/macros'] ([])
INFO:root:Caching type properties of properties = jenkins_jobs.modules.properties:Properties
INFO:root:Caching type scm of scm = jenkins_jobs.modules.scm:SCM
INFO:root:Caching type triggers of triggers = jenkins_jobs.modules.triggers:Triggers
INFO:root:Caching type builders of builders = jenkins_jobs.modules.builders:Builders
INFO:root:Caching type publishers of publishers = jenkins_jobs.modules.publishers:Publishers                                                                                  INFO:root:Caching type wrappers of wrappers = jenkins_jobs.modules.wrappers:Wrappers
INFO:root:Caching type parameters of parameters = jenkins_jobs.modules.parameters:Parameters
Traceback (most recent call last):
  File "/usr/bin/jenkins-jobs", line 10, in <module>
    sys.exit(main())
  File "/usr/lib/python3/dist-packages/jenkins_jobs/cli/entry.py", line 174, in main                                                                                              jjb.execute()
  File "/usr/lib/python3/dist-packages/jenkins_jobs/cli/entry.py", line 154, in execute
    ext.obj.execute(self.options, self.jjb_config)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/cli/subcommand/test.py", line 64, in execute
    builder, xml_jobs, xml_views = self._generate_xmljobs(options, jjb_config)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/cli/subcommand/update.py", line 119, in _generate_xmljobs
    xml_jobs = xml_job_generator.generateXML(job_data_list)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/xml_config.py", line 79, in generateXML
    xml_objs.append(self._getXMLForData(data))
  File "/usr/lib/python3/dist-packages/jenkins_jobs/xml_config.py", line 92, in _getXMLForData
    self._gen_xml(xml, data)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/xml_config.py", line 109, in _gen_xml
    module.gen_xml(xml, data)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/modules/publishers.py", line 8328, in gen_xml
    self.registry.dispatch("publisher", publishers, action)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/registry.py", line 286, in dispatch
    func(self, xml_parent, component_data)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/modules/publishers.py", line 4627, in postbuildscript
    registry.dispatch("builder", build_steps_xml, builder)
  File "/usr/lib/python3/dist-packages/jenkins_jobs/registry.py", line 288, in dispatch
    raise JenkinsJobsException(
jenkins_jobs.errors.JenkinsJobsException: Unknown entry point or macro 'role' for component type: 'builder'.
----------8<----------

Reverting to version 3.0.0 solves for now.