use Conan on Linux to install from system package manager

[kambala-decapitator]

This should avoid the need to list which packages to install and probably avoid giving different instructions for different package managers. Most probably would also work on macOS (Homebrew) and maybe Windows (msys2, choco).

see:

• tools.system.package_manager in https://docs.conan.io/2/reference/config_files/global_conf.html
• https://docs.conan.io/2/reference/tools/system/package_manager.html
• might also be useful https://docs.conan.io/2/reference/conanfile/methods/system_requirements.html