Make zstd a private dependency of LHCb

Currently Findzstd.cmake is not installed and thus not available in downstream projects. When building e.g. Lbcom, zstd seems to be found from the (LbEnv) conda environment, which is wrong.

zstd is in practice a private dependency of LHCb because its only user MDFLib is (almost) not dependend upon outside of LHCb.

In general, CMake looks for packages wherever CMake is installed (i.e. the conda environment) and so it's probably good to ignore that with something like this

get_filename_component(_CMAKE_INSTALL_DIR "${CMAKE_ROOT}" PATH)
get_filename_component(_CMAKE_INSTALL_DIR "${_CMAKE_INSTALL_DIR}" PATH)
list(APPEND CMAKE_SYSTEM_IGNORE_PREFIX_PATH "${_CMAKE_INSTALL_DIR}")

Not sure where this belongs exactly.

Follows up on !4444 (merged)

/cc @clemenci @cburr @frankm