s/nix style/v2-build/ in the documentation

I conjecture that mentioning Nix only adds confusion. Almost all people I know use `v2-build` (well actually they use `new-build`, but maybe `v2-build` is a bit better).

E.g.

- One of the primary innovations of Nix-style local builds is the distinction between local packages, which users edit and recompile and must be built per-project, versus external packages, which can be cached across projects.
+ One of the primary innovations of `v2-build` local builds is the distinction between local packages, which users edit and recompile and must be built per-project, versus external packages, which can be cached across projects.

Search the documentation for occurrences of “Nix-style” and inspect the surrounding terminology, including the example in the issue. Check the discussion before changing wording, since the proposed replacement is uncertain and the thread has been inactive for months; done means the agreed terminology is applied consistently.

Written by the indexing model from the issue text.