Allow custom vendored Node URL

[mlarraz]

Follow-up to #1406

Lets us ship custom builds, targeting different platforms than the Heroku default.

This is slightly more complicated than the Ruby change, as the URL contains a lot of platform-specific information, as well as interpolates the Node version.

My solution is to allow for a variable in the URL that gets substituted with the version.

This allows for dynamic lookups just like normal, and avoids hardcoding to a single version.

[edmorley]

(Switching code review request from the Languages team alias to the Ruby language owner)

[schneems]

The best way to install a custom node binary would be to make a micro buildpack https://devcenter.heroku.com/articles/buildpack-api. If which node returns zero it indicates to the Ruby buildpack that we do not need to install a node version. In the future we are moving away from installing node with CNBs (not available on Heroku yet) and instead relying on heroku/node buildpack for that functionality https://github.com/heroku/buildpacks-ruby.