stderr is always sent to stdout unless failOnStdErr is true

Describe the bug
A processes stderr is always sent to stdout instead of stderr unless the failOnStdErr option is set.

I want to silence stdout while keeping stderr. I tried to do this with this line:

const output = await getExecOutput('my-command', ['my-args'], {
  outStream: nullStream,
  errStream: process.stderr, // alternatively without this line
})

however, this unexpectedly silences both stdout and stderr.
Also setting the failOnStdErr option to true fixes the above problem, but also throws an error if the command prints to stderr, even when returning with a 0 exit code.

To Reproduce
Steps to reproduce the behavior:

1. Run this in a GitHub Action:

const nullStream = new stream.Writable({
  write(chunk: never, encoding: never, callback: never) {}
})

await exec('bash', ['-c', 'echo test 1>&2'], {
  outStream: nullStream,
})

2. Observe stdout and stderr

Expected behavior
The program should output 'test' to stderr

Actual behavior
There is no output

Start at the getExecOutput and exec stream-handling entry points, then reproduce the issue with the provided bash command and custom nullStream. Done means stdout can be suppressed while stderr remains visible, without requiring failOnStdErr or throwing for a successful command that writes to stderr.