Allow direnv to be installed via Devbox for integration

[kpervin]

What problem are you trying to solve?

Currently we need to install direnv outside of devbox.json for direnv to work. However, that seems antithetical to the entire purpose of devbox, as it means team members need to individually install direnv for it to work.

What solution would you like?

Add the means for devbox generate direnv to install direnv in devbox.json and then use that binary for all subsequent work.

Alternatives you've considered

Adding "eval \"$(direnv hook bash)\"" to init_hook in devbox.json is a workaround, but it means the shell is reloaded, causing hooks to run twice. For example:

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.13.6/.schema/devbox.schema.json",
  "shell": {
    "init_hook": [
      "eval \"$(direnv hook bash)\"",
      "echo 'Welcome to devbox!'"     
    ]
  }
}

$ devbox shell
Starting a devbox shell...
Welcome to devbox!
direnv: loading ~/projects/adaptiiv-iac/.envrc
direnv: using devbox
Welcome to devbox!
direnv: export +FOO