base: Make build log processing non-blocking.

We used to have 'build-derivations' write to the custom binary port
returned by 'build-event-output-port'.  However, custom binary ports
constitute continuation barriers, thereby preventing fibers from being
suspended.

To make build log processing non-blocking, we therefore invert this
inversion of control and use a suspendable I/O procedure,
'read-line/non-blocking', when reading the build log.

* src/cuirass/base.scm (read-line/non-blocking, process-build-log)
(build-derivations&): New procedures.
(%newline, build-event-output-port): Remove.
(spawn-builds): Use 'build-derivations&' instead of 'build-derivations'
with 'build-event-output-port'.