Always rebuild base and builder together

When we build either, it could pick up base image changes, such as
moving from stretch to buster. Make sure any time we build one we
build the other so that they stay in sync.

Change-Id: Ia28ad4f64114c88cc02289c9318a323ceb4f143d

.zuul.yaml

@@ -452,6 +452,7 @@
         - context: docker/python-builder
           repository: opendevorg/python-builder
     files: &python-builder_files
+      - docker/python-base/.*
       - docker/python-builder/.*
 
 - job:
@@ -479,6 +480,7 @@
           repository: opendevorg/python-base
     files: &python-base_files
       - docker/python-base/.*
+      - docker/python-builder/.*
 
 - job:
     name: system-config-upload-image-python-base