fix: properly promisify pipe stream args

[antongolub]

const p = $`echo "hello"`
  .pipe(
    new Transform({
      transform(chunk, encoding, callback) {
        callback(null, String(chunk).toUpperCase())
      },
    })
  )
  .pipe(fs.createWriteStream('/tmp/output2.txt'))
  
// p is a WriteStream instance with PromiseLike mixin
await p // is void

• Tests pass
• Appropriate changes to README are included in PR