manifests/jenkins-s2i: specify mirror for Jenkins plugins download

The new ITUP cluster for the RHCOS pipeline requires all connections
to be specified in a FirewallEgress file. Let's use the environment
variable `JENKINS_UC_DOWNLOAD`[1] to ensure the Jenkins plugins
are always downloaded from 'https://archives.jenkins.io'

[1]: https://github.com/openshift/jenkins/blob/ffc5df9d865e0c60947477a1ff7d354d1d618daf/2/contrib/jenkins/install-plugins.sh#L248

Co-authored-by: Aashish Radhakrishnan <[email protected]>

Author: 2 people

manifests/jenkins-s2i.yaml

@@ -72,6 +72,9 @@ objects:
             kind: ImageStreamTag
             name: ${JENKINS_S2I_SRC_IMAGESTREAM_NAME}
             namespace: ${JENKINS_S2I_SRC_IMAGESTREAM_NAMESPACE}
+          env:
+            - name: JENKINS_UC_DOWNLOAD
+              value: 'https://archives.jenkins.io'
           forcePull: true
       output:
         to: