also simplify environment prep and fix qemu dependencies
the matrix should ensure we never run out of space while building
unfortunately, github limits us to 10 inputs so we can't add a selector
for live iso platforms
for whatever reason, the old workflow was running out of space on the
runner. splitting it up into different jobs and combining it at the end
fixes that. not 100% optimal because it doesn't reüse the ROOTFS when
building PLATFORMFS and PLATFORMFS when building SBC images. I couldn't
think of a good way to do that if e.g. generating ROOTFSes was not
done in the ROOTFS step.
also add checkboxes for each kind of image to make it more ergonomic to
generate a subset of images while still keeping the default strings.
unfortunately, this means we're at the max number of inputs so something
will have to change if we reënable netboot images or something.