Bug 1381770 - Use $WORKSPACE instead of $HOME/workspace in tooltool-download.sh. r=mshal

All the current users of tooltool-download.sh set $WORKSPACE. This will
allow to reuse the script on different types of workers, that don't have
$WORKSPACE set to $HOME/workspace, but still have the source in
$WORKSPACE/build/src.

--HG--
extra : rebase_source : c91542da70109e5708a009542a0f588e30b541a9

taskcluster/scripts/misc/tooltool-download.sh

@@ -1,6 +1,6 @@
 # Fetch a tooltool manifest.
 
-cd $HOME/workspace/build/src
+cd $WORKSPACE/build/src
 
 : TOOLTOOL_CACHE                ${TOOLTOOL_CACHE:=/home/worker/tooltool-cache}
 export TOOLTOOL_CACHE