g:ale_filename_mappings doesn't work for elixir-ls

## Information

**VIM version**
NVIM v0.4.4
Build type: Release

Operating System:
ArchLinux
## What went wrong

With the following config:
```vim
let g:ale_filename_mappings = {
\ 'elixir-ls': [
\ ['/home/dannpg/Work/project', '/opt/app']
\ ]
\}
```

I see the following output from elixir-ls:
```
{"jsonrpc":"2.0","method":"window/logMessage","params":{"message":"\n10:29:34.893 [error] GenServer ElixirLS.LanguageServer.Server terminating\n** (File.Error) could not set current working directory to \"/home/dannpg/Work/project\": no such file or directory\n (elixir 1.10.3) lib/file.ex:1523: File.cd!/1\n (language_server 0.6.2) lib/language_server/serve ...
```

If I set:

```vim
let g:ale_lsp_root = {'elixir-ls': '/opt/app'}
```

Then `elixir-ls` operates OK, but it reports the docker paths (/opt/app), so Vim is unable to jump to the correct file.

### :ALEInfo
```
Current Filetype: elixir
Available Linters: ['credo', 'dialyxir', 'dogma', 'elixir-ls', 'mix']
Enabled Linters: ['credo', 'elixir-ls']
Ignored Linters: []
Suggested Fixers:
'mix_format' - Apply mix format to a file.
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
Linter Variables:

let g:ale_elixir_elixir_ls_config = {'elixirLS': {'dialyzerEnabled': v:false}}
let g:ale_elixir_elixir_ls_release = '.vim-config/elixir-ls-wrappers'
Global Variables:

let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = 0
let g:ale_command_wrapper = ''
let g:ale_completion_delay = v:null
let g:ale_completion_enabled = 0
let g:ale_completion_max_suggestions = v:null
let g:ale_disable_lsp = 0
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 1
let g:ale_fixers = {'elixir': ['mix_format'], 'ruby': ['rufo'], 'javascript': ['prettier', 'eslint'], 'rust': ['rustfmt']}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'normal'
let g:ale_linter_aliases = {}
let g:ale_linters = {'elixir': ['credo', 'elixir-ls'], 'ruby': ['rubocop', 'ruby'], 'typescript': ['tsserver']}
let g:ale_linters_explicit = 0
let g:ale_linters_ignore = {}
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_lsp_root = {}
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = -1
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 1
let g:ale_sign_error = '>>'
let g:ale_sign_info = '--'
let g:ale_sign_offset = 1000000
let g:ale_sign_style_error = '>>'
let g:ale_sign_style_warning = '--'
let g:ale_sign_warning = '--'
let g:ale_sign_highlight_linenrs = 0
let g:ale_statusline_format = v:null
let g:ale_type_map = {}
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 0
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:

(executable check - success) mix
(finished - exit code 0) ['/bin/zsh', '-c', 'cd ''/home/dannpg/Work/project'' && mix help credo && mix credo suggest --format=flycheck --read-from-stdin ''/home/dannpg/Work/project/lib/app/model.ex'' < ''/tmp/nvimdGKJSV/3/payments.ex''']

<<>>
# mix credo

Run code analysis (use `--help` for options)

Location: _build/dev/lib/credo/ebin
<<>>

(executable check - success) .vim-config/elixir-ls-wrappers/language_server.sh
```