This prevents us from needing to recompile the dependencies when that
environment variable changes, which generally changes on every commit,
which is far more frequently than the dependencies are actually changed.
Users of the nix package can now just use `.override` to choose what
features they want.
This also makes RocksDB automatically use jemalloc when Conduit is
configured to use jemalloc.
Matthias Ahouansou
Charles Hall