The licence key it needs here is not the same thing as the licence that the subscriptions talk about, it's a signing key:
I think this is only required to build X-pack, but I am not sure how to go about building just the non-X-pack components any more.
Why do you want to produce your own release build? The only real difference is the -SNAPSHOT in the version number, and I think it's important to use this to see the difference between official versions (built in a very carefully controlled environment) and versions built elsewhere.