jenkins: Use a single PV for all Buildroot jobs

Instead of allocating a volume for each individual Buildroot-based
project, I think it will be easier to reuse the same one for all of
them.  It's not like we can really run more than one job at a time,
anyway.

jenkins/workspace-volume.yaml

@@ -1,10 +1,10 @@
 apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
-  name: buildroot-airplaypi
+  name: buildroot
   namespace: jenkins-jobs
   labels:
-    app.kubernetes.io/name: buildroot-airplaypi
+    app.kubernetes.io/name: buildroot
     app.kubernetes.io/component: jenkins
 spec:
   accessModes: