Add the Unnoficial Builds values

[nschonni]

There are now a separate release chain of "unofficial builds" for some non-Tier 1 release plat forms https://github.com/nodejs/unofficial-builds. It also has it's own index.json that can be found here https://unofficial-builds.nodejs.org/download/release/index.json

[chicoxyzzy]

There's no plans to add unofficial builds to the repo anymore