use better tmp dir

setup-node-ubuntu.bash

@@ -9,7 +9,7 @@ set -u
 set -o pipefail
 
 if [ -z "${my_tmp-}" ]; then
-  my_tmp=$(mkdir -p)
+  my_tmp="$(mktemp -d -t node-installer.XXXXXXXX)"
 fi
 if [ -z "${PREFIX-}" ]; then
   PREFIX=""