Disable codenarc for CI builds

CodeNarc appears to hang in drone.io.
This apparently happened before, resulting in us excluding the codenarcMain and codenarcTest tasks
from running in drone.io.  However, when I added browser tests for ratpack-pac4j, CodeNarc hung on
that source set.  Thus, a more maintainable solution: skip all codenarc tests when the ciBuild flag is set.