Adapt swh-apps build to allow for the rebuild of a base image

This requires [1] which reworks the image build logic to share a common base image.

To clarify, it requires at least the commit with the base image definition, not necessarily the all mr.

Depends partially on [1]

[1] swh/infra/swh-apps!51 (merged)

Refs. swh/infra/sysadm-environment#5560 (closed)

jobs/templates/swh-apps-build-docker-images.groovy.j2

@@ -75,6 +75,9 @@ pipeline {
     }
 
     stage('Build/Reuse "swh-base" image') {
+      when {
+        expression { rebuild_base_image }
+      }
       steps {
         script {
             build job: 'build-docker-image',