Use a non-blocking socket for store communication.

Set the store socket as non-blocking so that fibers communicating with the
store don't get blocked as described here: https://issues.guix.gnu.org/43565.

* src/cuirass/base.scm (with-store): Set the store socket as non-blocking.
(build-derivations&): Unset current-read-waiter and current-write-waiter.