[Driver][test] Make RISCV tests robust with PATH=

When `riscv64-unknown-linux-gnu-ld` is in the PATH, `clang -### -fuse-ld=ld --target=riscv64-unknown-linux-gnu` will use unknown-linux-gnu-ld first, which causes the error in the lit test.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D127589