Automatically delete $TEMP_PATH if it's empty

bin/ruby-build

@@ -27,6 +27,8 @@ build_failed() {
   { echo
     echo "BUILD FAILED"
     echo
+
+    if ! rmdir "${TEMP_PATH}" 2>/dev/null; then
       echo "Inspect or clean up the working tree at ${TEMP_PATH}"
 
       if file_is_not_empty "$LOG_PATH"; then
@@ -35,6 +37,7 @@ build_failed() {
         echo "Last 10 log lines:"
         tail -n 10 "$LOG_PATH"
       fi
+    fi
   } >&3
   exit 1
 }