Adding the following lines to the bottom of the script sorted the issue:
[ -d $HOME/bin ] || mkdir $HOME/bin
[ -f $HOME/bin/karma ] || ln -s /scratch/jenkins/addons/node/$node_name/bin/karma $HOME/bin/karma
[ -f $HOME/bin/node ] || ln -s /scratch/jenkins/addons/node/$node_name/bin/node $HOME/bin/node