use bash based on env, instead of hard coding location

speed-test-ttfb.sh

@@ -1,4 +1,5 @@
-#!/bin/bash
+#!/usr/bin/env bash
+
 # speed-test-ttfb https://www.example.com/
 
 curl -w "Connect time: %{time_connect} Time to first byte: %{time_starttransfer} Total time: %{time_total} \n" -s -o /dev/null "$@"