These take up 2 GiB every time anything in the minimal installer
changes, or up to 4 GiB per day. We already stopped building Amazon
images in 9426d90c67. Meaningful
installer changes are rare enough, and the couple of days it takes
for them to trickle down to the large channel acceptable enough,
that this is mostly a waste of space.
This should buy enough slack to build `stdenv` on `staging` without
contributing to cache size growth.
862911fd22