Fast-path _expand_location_for_build_script_runner (#3637)

Before:
<img width="1172" height="180" alt="image"
src="https://github.com/user-attachments/assets/00417d21-2a1c-439f-9aa7-85635f0741ac"
/>

After:
<img width="1159" height="177" alt="image"
src="https://github.com/user-attachments/assets/7925646c-420e-4db0-9177-d4b08fc7bef0"
/>

Author: dzbarsky

rust/private/utils.bzl

@@ -270,6 +270,11 @@ def _expand_location_for_build_script_runner(ctx, env, data, known_variables):
     Returns:
         string: The location-macro expanded version of the string.
     """
+
+    # Fast-path - both location expansions and make vars have a `$` so we can short-circuit everything.
+    if "$" not in env:
+        return env
+
     for directive in ("$(execpath ", "$(location "):
         if directive in env:
             # build script runner will expand pwd to execroot for us