MB-9767:  add build/ directory to set of files to ignore.

The Jenkins job

    http://factory.hq.couchbase.com:8080/job/testrunner-gerrit-master

runs the script from the build repo:

    build/scripts/jenkins/commit_validation/testrunner-gerrit-master.sh

which replaces the build directory in the jenkins WORKSPACE with a clone
of the build repo, and then runs the testrunner-gerrit-master.sh script.

Any files saved here will be overwritten in that environment.

Change-Id: I375a9553af705a6f11c04878f655432d5e2c4a08
Reviewed-on: http://review.couchbase.org/31253
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>