build: allow proper generation of html docs

`gen-doc` always calls `gen-json`, which means it's impossible to
generate html docs. Changed this to pass in the command the user wants
to run.

PR-URL: https://github.com/nodejs/node/pull/14932
Fixes: https://github.com/nodejs/node/issues/14930


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>