Saving in non-mix project clears diagnostics

**Expert version (expert -v)**
Expert v0.1.0-b237fd5

**Steps to reproduce**

`min.lua`
```lua
vim.lsp.config('expert', {
cmd = { '/path/to/expert', '--stdio' },
root_markers = { 'mix.exs', '.git' },
filetypes = { 'elixir', 'eelixir', 'heex' },
})

vim.lsp.enable 'expert'
```

1. In a git repo, open elixir file with `nvim -u min.lua` containing:

```elixir
defmodule Test do
def execute() do
:foo + 1
end
end
```

Notice the diagnostic:

```
evaluation of operator '+'/2 will fail with a 'badarith' exception
```

2. Save in neovim with `:w`.
3. The dianostic then disappears.

The described behavior isn't observed with other LS.