jenkinsfile: checkout gerrit_refspec

Change-Id: I79991fe7c84b395e69db41721e473376740cb797

extras/ci/client-qt-gnulinux/Jenkinsfile

@@ -56,6 +56,7 @@ pipeline {
             steps {
                 script {
                     sh """
+                        git fetch ssh://sblin@review.jami.net:29420/jami-client-qt $GERRIT_REFSPEC && git checkout FETCH_HEAD
                         git rev-parse HEAD
                         git submodule update --init --recursive
                     """