This reduces the wall-clock time of:
./pre-inst-env guix system vm gnu/system/examples/desktop.tmpl -n
from 2m13s to 53s (the timings depend on which derivations have already
been built and are in store; in this case, many were missing).
* guix/store.scm (default-cutoff): New variable.
(map/accumulate-builds): Use it. Parameterize it in recursive calls to
have decaying cutoff.
