elixir: Make files required by elixir-ls executable (#16819)

Closes:

- #15802

This PR fixes an issue in the `elixir-ls` language server installation
where some of the required scripts was not being made executable when
installed from GitHub.

Release Notes:

- Fixed elixir-rs files not being executable ([#15802](https://github.com/zed-industries/zed/issues/15802))