ci: Fix file ownership before archiving

Several log files in `/var/log` are owned by users other than root.
From outside the user namespace, these files are inaccessible to the
unprivileged user, so we need to change their ownership before we can
archive them.

ci/Jenkinsfile

@@ -60,6 +60,7 @@ pipeline {
 
     post {
         failure {
+            sh 'unshare -Ur --map-auto chown root:root -R tmp log'
             dir('tmp/portage') {
                 archiveArtifacts '*/*/temp/*.log'
             }