cmake: using MD5_INFILE variable instead of ZephyrPackagePath.txt

Running `west zephyr-export` creates an entry in the CMake User package
registry.

During this process, a temporary file `current_path.txt` is created.

This commit ensure the file is removed when no longer needed.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>