[tests] Find bash on PATH in the dart:io test scripts

Closes #64553

GitOrigin-RevId: 13a90ed
Change-Id: I4db4c35ddf59b8cffcae2eb86b305d207a5f55ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/562360
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

Author: pattems

tests/standalone/io/dart_std_io_pipe_test.sh

@@ -1,4 +1,4 @@
-#! /bin/bash
+#!/usr/bin/env bash
 
 # This script expects the following arguments
 # $1: Path to dart executable

tests/standalone/io/regress_54516_test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 
 echo $0