add new 'sudo=' Build field

This introduces a new per-Build field called sudo which is a script like init or prebuild. It is only run on the buildserver VM. It is run first, before anything else. And it is run as root using sudo. If someone runs fdroid build without a buildserver on a Build that includes sudo=pip3 install mymodule, the script "pip3 install mymodule" will be printed out in a warning to the user running the build.

This is WIP since its based on !296 (merged)

@mimi89999 @relan @est @krt