ci: Update workspace volume PVC name

Now that we have _democratic-csi_ for storage management, the old manual
iSCSI volumes are being replaced with dynamically provisioned volumes.
As part of the migration process, I decided to consolidate all of the
Buildroot Jenkins jobs' workspaces into a single huge PVC.  This way, I
don't have to define a PVC for every one individually.

ci/Jenkinsfile

@@ -12,7 +12,7 @@ pipeline {
         kubernetes {
             yamlFile 'ci/podTemplate.yaml'
             workspaceVolume persistentVolumeClaimWorkspaceVolume(
-                claimName: 'buildroot-airplaypi'
+                claimName: 'buildroot'
             )
             defaultContainer 'build'
         }