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

Cargo.lock

@@ -9318,16 +9318,6 @@ dependencies = [
  "syn 2.0.48",
 ]
 
-[[package]]
-name = "subst"
-version = "0.3.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ca1318e5d6716d6541696727c88d9b8dfc8cfe6afd6908e186546fd4af7f5b98"
-dependencies = [
- "memchr",
- "unicode-width",
-]
-
 [[package]]
 name = "subtle"
 version = "2.5.0"
@@ -9575,7 +9565,6 @@ dependencies = [
  "serde_json_lenient",
  "sha2 0.10.7",
  "shellexpand",
- "subst",
  "util",
 ]