ZED_WORKTREE_ROOT should always point to a directory

[notpeter]

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• Completed

Describe the bug / provide steps to reproduce it

Steps to reproduce:

1. Open your Zed tasks.json and add the following:

   {
     "label": "zed env",
     "command": "env | grep ZED"
   },
   

2. Cmd-P, task: spawn, select zed env at the bottom of the list.
3. Note:

   ZED_WORKTREE_ROOT=/Users/peter/.config/zed/tasks.json
   

Documentation suggests this variable will point to a folder, which I believe makes sense:

zed/docs/src/tasks.md

Line 84 in 86f5bb1

- `ZED_WORKTREE_ROOT`: absolute path to the root of the current worktree. (e.g. `/Users/my-user/path/to/project`)

Proposal: Behavior of ZED_WORKTREE_ROOT should match that default behavior of cwd which was improved in #22004 to not point at a file when spawned from a buffer that is outside of worktree (like the tasks.json example above).

This makes it possible to spawn other applications into the correct working directory, even when that must be passed an argument, not just implicitly inherited from cwd/$PWD.

For example:

  {
    "label": "lazygit",
    "command": "alacritty -o 'window.startup_mode=\"SimpleFullscreen\"' --working-directory ${ZED_WORKTREE_ROOT} --command lazygit",
    "hide": "always"
  },

This does not work: cd /tmp; alacritty (working directory defaults to ~)
This does work: alacritty --working-directory=/tmp

Originally reported by @VOID404 here:

• Tasks unexpectedly try to use the tasks.json file (or the keymap.json file) as a **current working directory** (it's not a directory, it's a file!!) #20060 (comment)

Zed Version and System Specs

Zed: v0.170.0 (Zed Nightly af1a3cb)
OS: macOS 14.7.0
Memory: 64 GiB
Architecture: aarch64

If applicable, add screenshots or screencasts of the incorrect state / behavior

No response

If applicable, attach your Zed.log file to this issue.

N/A