task: Fix variable substitution for free variables (#10434)

Fixes regression from https://github.com/zed-industries/zed/pull/10341
where it was not possible to use non-zed environmental variables (e.g.
$PATH) in task definitions.

No release note, as this didn't land on Preview yet.
Release Notes:

- N/A

crates/task/Cargo.toml

@@ -19,7 +19,6 @@ serde.workspace = true
 serde_json_lenient.workspace = true
 sha2.workspace = true
 shellexpand.workspace = true
-subst = "0.3.0"
 util.workspace = true
 
 [dev-dependencies]