Bug 1289249 - Ensure cwd is /home/worker; r=dustin

When we switch to run-task in a subsequent commit, we can no longer
rely on cwd being /home/worker. So have build-linux.sh change the cwd
explicitly.

MozReview-Commit-ID: 5sYAjHCFi3s

--HG--
extra : rebase_source : 35e1ceafb6dc880f7f28ee152ae7c78d67ca2f29

taskcluster/scripts/builder/build-linux.sh

@@ -107,6 +107,8 @@ if [ -n "$MOZHARNESS_OPTIONS" ]; then
     done
 fi
 
+cd /home/worker
+
 python2.7 $WORKSPACE/build/src/testing/${MOZHARNESS_SCRIPT} ${config_cmds} \
   $debug_flag \
   $custom_build_variant_cfg_flag \