Fix project environment not working correctly with multiple worktrees (#22246)

Fixes https://github.com/zed-industries/zed/issues/21972

This fixes two bugs:

**Bug 1**: this bug caused us to only ever load a single environment in
a multi-worktree project, thanks to this line:

```rust
if let Some(task) = self.get_environment_task.as_ref()
```

We'd only ever run a single task per project, which is wrong.

What does code does is to cache the tasks per `worktree_id`, which means
we don't even need to cache the environments again, since we can just
cache the `Shared<Task<...>>`.

**Bug 2**: we assumed that every `worktree_abs_path` is a directory,
which lead to `Failed to run direnv` log messages when opening a project
that had a worktree with a single file open (easy to reproduce: open a
normal project, open your settings, close Zed, reopen it — the settings
faile caused environments to not load)

It's fixed by checking whether the `worktree_abs_path` is an absolute
directory. Since this is always running locally, it's fine to use
`smol::fs` here instead of using our `Fs`.

Release Notes:

- Fixed shell environments not being loaded properly to be used by
language servers and terminals in case a project had multiple worktrees.
- Fixed `Failed to run direnv` messages showing up in case Zed restored
a window that contained a worktree with a single file.
https://github.com/zed-industries/zed/issues/21972

crates/project/src/project.rs

@@ -1207,13 +1207,6 @@ impl Project {
                 .await
                 .unwrap();
 
-            project.update(cx, |project, cx| {
-                let tree_id = tree.read(cx).id();
-                project.environment.update(cx, |environment, _| {
-                    environment.set_cached(&[(tree_id, HashMap::default())])
-                });
-            });
-
             tree.update(cx, |tree, _| tree.as_local().unwrap().scan_complete())
                 .await;
         }