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.
I'm a Makefile novice, so please let me know if there's a better way to
do this :)
Fixes: https://github.com/nodejs/node/issues/14930
Checklist
-
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
build