Install default plans under MSCCLPP_CACHE_DIR/default (#769)

### Summary
Update the installer to place bundled default execution plans under
`<MSCCLPP_CACHE_DIR>/default`, which is where the runtime already looks
for bundled plans.

### Background
The C++ runtime treats `MSCCLPP_CACHE_DIR` as the cache *root* and loads
bundled default plans from `<cache root>/default`.
When `MSCCLPP_CACHE_DIR` was set, the installer instead wrote bundled
plans
directly into the cache root, causing the runtime to miss them.

This surfaced while running benchmarking tests with a non-default
`MSCCLPP_CACHE_DIR`, where the bundled plans were not being discovered.

### Change
This PR updates the installer to always install bundled default plans
into
`<MSCCLPP_CACHE_DIR>/default`, preserving the existing runtime contract.

### Scope
- Installer-only change
- No runtime behavior changes

### Validation
Manual inspection of the updated install path.
Successful build

---------

Co-authored-by: Ekow Wellington <t-ekoww@microsoft.com>

python/mscclpp/__main__.py

@@ -57,7 +57,7 @@ default_algo_configs = [
 
 
 def create_default_plans():
-    plan_dir = os.environ.get("MSCCLPP_CACHE_DIR", Path.home() / ".cache/mscclpp/default")
+    plan_dir = os.path.join(os.environ.get("MSCCLPP_CACHE_DIR", Path.home() / ".cache/mscclpp"), "default")
     plan_path = Path(plan_dir)
     if plan_path.exists():
         shutil.rmtree(plan_path)