[Clang] Propagate 'SystemDrive' environment variable for unit tests (#90478)

On Windows, running unit tests by directly invoking `llvm-lit.py` (e.g.
`python3 llvm-lit.py clang/test/Unit`) may create a folder named `%SystemDrive%`
in the current working directory. This appears to happen because `lit.cfg.py` in 
`clang/test/Unit` does not propagate the `SystemDrive` environment variable.