ci: try fix builds

This chmod is combined with "Clean before checkout" behaviour
that deletes everything, except .git. This should give us
the sufficient ordering to ensure that everything has the
correct permissions.

Story so far:

Checkout with "Wipe out repository & force clone" didn't work,
because the chmod would've run after trying to delete.

Deleting as a post action doesn't work, because during timeouts
the always steps don't get executed sometimes.

We cannot easily run docker with non root, due to how our ci
image works.

Change-Id: Id4c1605d39ae8d7722ba3dd1ab5df8618de3309b

Jenkinsfile.public

@@ -19,6 +19,7 @@ pipeline {
     stages {
         stage('Checkout') {
             steps {
+                sh "chmod -R 777 ." // ensure Jenkins can delete the working directory, with before-checkout hook
                 checkout scm
 
                 sh 'mkdir -p .build'