### 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>
