- isohybrid is available in our Vagrant build VMs. The manual build
   documentation is totally outdated and broken, nobody complained about it,
   which suggests nobody tried to follow it since 1+ years.
   And anyway, if isohybrid is not installed, the build will fail loudly
   later on.

 - $LB_BINARY_IMAGES is correctly set because auto/config passes
   "--binary-images iso" to "lb config". Checking it here brings no value and
   just makes things harder for anyone who would want to add support for other
   kinds of images.

 - It's been a while since we've stopped releasing source tarballs :) One can't
   reach auto/config via "rake build" if one is not building from Git, and
   building without rake is unsupported (good luck with that).