Fix pin images task with atmosphere_image_prefix

This change also includes a skip for running jobs when any changes
in the `build` folder are done.

Change-Id: I632a2d71beb4227f9067ed936550a714df269232
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index 9e9da00..ce8ba97 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -71,6 +71,7 @@
     run: zuul.d/playbooks/molecule/run.yml
     post-run: zuul.d/playbooks/molecule/post.yml
     irrelevant-files:
+      - ^build/
       - ^doc/
       - ^releasenotes/
     vars: