cp.exec decide to use `_stdout`(_stdout is string) or
`Buffer.concat(_stdout)`(_stdout is buffer array) by options.encoding.
but std(out|err) encoding can be changed. If encoding is changed to
not null, `_stdout` will become string, and `Buffer.concat(_stdout)`
will throw TypeError. This patch will fix it, use
options.encoding and `std(out|err)._readableState.encoding`.
PR-URL: https://github.com/nodejs/node/pull/18976
Fixes: https://github.com/nodejs/node/issues/18969
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
