Cache Elixir builds for each version
Cache Elixir builds for each version

Building each Elixir version takes over a minute. To mitigate this,
this commit uses Semaphore's `cache` to store Elixir builds for a
given Elixir and Erlang version between CI runs.

appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir