tools: make doctool work if no internet available

Allow doctool to fallback to use local files if not building a release
build.

Backport-PR-URL: https://github.com/nodejs/node/pull/32642
PR-URL: https://github.com/nodejs/node/pull/30214
Fixes: https://github.com/nodejs/node/issues/29918


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>