Bug 1532747 - Use 'mach python' with repackage.sh script, CLOSED TREE, r=catlee

Differential Revision: https://phabricator.services.mozilla.com/D26239

--HG--
extra : moz-landing-system : lando

taskcluster/scripts/builder/repackage.sh

@@ -92,7 +92,7 @@ fi
 
 cd /builds/worker
 
-python2.7 $WORKSPACE/build/src/testing/${MOZHARNESS_SCRIPT} \
+$GECKO_PATH/mach python $GECKO_PATH/testing/${MOZHARNESS_SCRIPT} \
   ${config_path_cmds} \
   ${config_cmds} \
   $actions \