Add a question at the end of "pmbootstrap init", to ask if the user
wants to build outdated packages during "pmbootstrap install". Store the
result in the new pmbootstrap.cfg key "install_build_pkgs". I've put it at
the end, because it is a rather complicated question compared to the rest.
This is useful to speed up the installation for casual users who can now
avoid compiling packages. But also for the official images where we only
want to ship the official binary packages and not build anything
on-the-fly.
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