Support use of relative path for `livebook`

Hi!

I'm using devbox and I have livebook installed at ./.nix-mix/escripts but I can't get the extension to run livebook from there (without using a full path. I've tried setting code-livebook.livebookExecutablePath to

${workspaceFolder}/.nix-mix/escripts/livebook
.nix-mix/escripts/livebook
livebook (it's on my path)

None worked.

This is not related to devbox, but in case it makes it easier for you to the debug the issue, here's my devbox.json

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.13.0/.schema/devbox.schema.json",
  "packages": [
    "elixir@latest",
    "bash@latest",
    "zsh@latest",
    "xz@latest",
    "zig@latest",
    "_7zz@latest",
    "erlang@latest",
    "lexical@latest"
  ],
  "env": {
    "MIX_HOME":               "$PWD/.nix-mix",
    "HEX_HOME":               "$PWD/.nix-hex",
    "ERL_AFLAGS":             "-kernel shell_history enabled",
    "SPACESHIP_PROMPT_ASYNC": "false"
  },
  "shell": {
    "init_hook": [
      "mkdir -p .nix-mix",
      "mkdir -p .nix-hex",
      "mix local.hex --force",
      "mix local.rebar --force",
      "export PATH=\"$PATH:$MIX_HOME/escripts\""
    ],
    "scripts": {
      "test":  "mix run",
      "build": "mix burrito.build --release uisl"
    }
  }
}

JonasGruenwald / code-livebook

Support use of relative path for `livebook` #1