2025-02-01

use legacy driver when generating index store output

Summary:
The swift-driver enforces output paths are unique, and will fail when producing index store output with:
```
SwiftDriverExecution/MultiJobExecutor.swift:207: Fatal error: multiple producers
for output /dev/null: Compiling ...
```
For now force the legacy driver for index store output.

Reviewed By: drodriguez

Differential Revision: D68978696

fbshipit-source-id: 38c94f3202abbf13cb7cf07758ceab5c63462349