src: fix NODE_OPTIONS parsing bug

I, uhm, might have messed up by using a `substr(start, end)`
signature when `std::string` actually uses `substr(start, len)`.
Fix that.

Fixes: https://github.com/nodejs/node/issues/22526
Refs: https://github.com/nodejs/node/pull/22392

PR-URL: https://github.com/nodejs/node/pull/22529


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>