set TheP8I paths and set PYTHIA8PATH during Pythia8_i compilation

This MR removes the dependence on the hardcoded paths of TheP8I package, which is now found in the ATLAS externals. The Generators/Herwig7_i/share/common/Herwig7_TheP8I.py script can be included in any JOs script to hadronise events using the Pythia8 hadronisation model. This relies on the PYTHIA8PATH environment variable to be set during compilation, which is done through the Generators/Pythia8_i/Pythia8EnvironmentConfig.cmake file.