Feature request: make searching for ASDF optional

I'm in an environment where we use Nix very heavily to override everything, and while I do have asdf installed, it turns out to, most of the time, not be the versions of Elixir/Erlang that I need.

I fixed it locally by changing this line in `launch.sh` to have a bogus path to search for ASDF :

`ASDF_DIR=${ASDF_DIR:-"${HOME}/.asdfx"}`

There may be a larger decision over whether elixir-ls should do this at all, or whether the paths to the executable should be assumed to be known in the environment at launch time.

In emacs for instance, for Nix reasons, I am using direnv which triggers nix, so I use this:
https://github.com/purcell/envrc

Basically it runs direnv for a project when I open the first buffer (and it reuses the values for each subsequent open buffers), and that gives me things like a custom PATH for each project, so emacs knows the correct version of elixir and erlang for the given buffer.