[mlri][ExecutionEngine] Don't globaly set CMake prefixes to find HIP

The HIP CMake files expect to find their own dependencies and don't
use or respect PATHS or HINTS, relying on CMAKE_PREFIX_PATH to contain
/opt/rocm and /opt/rocm/hip . This is not great for the rest of the
build. Therefore, copy the CMake prefix path, add the ROCm
directories, find HIP, and reset the path to its old value.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D142391