[RISCV][test] Make PATH empty when testing --gcc-toolchain is multilib_riscv_elf_sdk

Due to D79842, clang dirver would search possible tool name in both possible
locations, then moving to the next name. The gcc toolchain `llvm-project/clang/test/Driver/
Inputs/multilib_riscv_elf_sdk` don't have a `riscv64-unknown-elf-ld` executable in
`llvm-project/clang/test/Driver/Inputs/multilib_riscv_elf_sdk/bin/`. So when searching
`riscv64-unknown-elf-ld`, if there is a `riscv64-unknown-elf-ld` in `PATH`, the
test would fail.

This patch makes the `PATH` empty when testing it.

Differential Revision: https://reviews.llvm.org/D122588