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

[pattems]

tests/standalone/io/dart_std_io_pipe_test.sh and tests/standalone/io/regress_54516_test.sh start with #!/bin/bash, so dart_std_io_pipe_test and regress_54516_test fail on systems where bash isn't installed at /bin/bash, such as NixOS.

This change switches both shebangs to #!/usr/bin/env bash. Systems with /bin/bash behave the same as before.

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[copybara-service]

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[copybara-service]

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