Added config option to force build

[KennethLj]

This is to support the recommendation that was printed to the console when the binary download failed:

You can force the project to build from scratch with:

config :rustler_precompiled, :force_build, folio: true

In order to force the build, you also need to add Rustler as a dependency in your mix.exs:

{:rustler, ">= 0.0.0", optional: true}