Fix CWD is not correctly respected when appending child paths

Right now when you add `"cwd": "$ZED_WORKTREE_ROOT/some/path"` to your config,
it won't work as expected. Because we only added the `cwd` to the task template when the cwd was a valid path.
But when you add `"cwd": "$ZED_WORKTREE_ROOT` to your config, it will never be a valid path, so the `cwd` will never be added to the task template and fallback to the `$ZED_WORKTREE_ROOT` already.

So by just always adding it to the task template, we will try to resolve it and otherwise fallback to the `$ZED_WORKTREE_ROOT`.

crates/task/src/debug_format.rs

@@ -185,7 +185,7 @@ impl DebugTaskDefinition {
             command,
             args,
             task_type,
-            cwd: if cwd.is_some() { self.cwd } else { None },
+            cwd: self.cwd,
             ..Default::default()
         })
     }