start-regtest: use env-based bash shebang

Find bash via PATH rather than hard-coding /bin/bash, for systems
(NixOS, macOS w/ homebrew bash) where the system bash isn't current.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

start-regtest

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 print_success() {
   printf "\033[;1;32mPASSED\033[;0m $1\n"
 }