Coerce gets called twice for nested command positionals

[CupOfTea696]

Coerce gets called multiple times for nested command positional, this could cause issues when the arg type is converted.

Example Script:

import yargs from 'yargs'

const argv = yargs('rule add foo bar baz').showHelpOnFail(false).fail(msg => {
  console.error(`[ERR] ${msg}`)
  process.exit(1)
}).command('rule', '', yargs => {
  return yargs.command('add <name> <desc...>', '', yargs => {
    return yargs.positional('desc', {
      type: 'string',
      coerce: arg => {
        console.log(arg)

        return arg.join(' ')
      }
    })
  })
}).argv

// the parsed data is stored in argv.
console.log(argv)

Expected Output:

[ 'bar', 'baz' ]
{ _: [ 'rule', 'add' ], '$0': 't.js', name: 'foo', desc: 'bar baz' }

Actual Output:

[ 'bar', 'baz' ]
bar baz
[ERR] arg.join is not a function

[shadowspawn]

This was fixed in #1978. Thanks @CupOfTea696 for report, and fix from @jly36963