build.sbt: enable forking for all tasks

This should fix running builds on a CI server, where you typically
perform a sequence of sbt commands such as:

    $ ./sbt clean test scoverage:test package doc assembly

Without forking enabled for all tasks (instead of only forking when
running tests) a full build will fail most of the time.
Fix generation of path to temporary Kakfa data log directory (log.dirs)