Cleaner way to find out where tracemonkey is.

scripts/build.sh

@@ -28,7 +28,7 @@ pip install -q -r requirements.txt
 git submodule sync -q
 git submodule update --init
 
-export SPIDERMONKEY_INSTALLATION="/usr/local/bin/tracemonkey"
+export SPIDERMONKEY_INSTALLATION=`which tracemonkey`
 
 echo "Starting tests..." `date`