ci: Use persistent volumes for portage, binpkgs

Recompiling the entire OS for every build takes way too long for
development purposes.  Since we build binary packages anyway, let's keep
them around between builds by storing them on a Kubernetes persistent
volume.

Eventually, I want add a build parameter to wipe out the binary package
storage to force a from-scratch build.  We'll use that option for robust
periodic builds, once development has slowed down.

ci/Jenkinsfile

@@ -9,6 +9,12 @@ pipeline {
     }
 
     stages {
+        stage('Prepare') {
+            steps {
+                sh '. ./prepare.sh'
+            }
+        }
+
         stage('Build') {
             steps {
                 sh 'env -i PATH="${PATH}" make -C aimee-os CONFIGDIR=${PWD} O=/build'