build: run `npm install` for doc builds in tarball

Run `npm install` before building the documentation from release
tarballs. The doctool currently depends on `js-yaml`, which
is imported from the `tools/eslint` subtree; however, release
tarballs don’t contain that directory.

Running `npm install` is clearly not a beautiful solution,
but it works.

Fixes: https://github.com/nodejs/node/issues/7872
PR-URL: https://github.com/nodejs/node/pull/8413


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>