Fix Python API CMake file ETHOS_U_DRIVER_LIB path

The ETHOS_U_DRIVER_LIB environment variable in the Python API CMake file
specifies that the driver library archive will be found in a lib
subdirectory in the build directory. This is incorrect as the archive
will be put directly into the driver library directory and the CMake
file has been updated accordingly.

Change-Id: Ie296f89f7669d69f193d96d994ad8e1918e3d1eb