Specify bazelisk_targets as a list, and join the targets as
space-separated in the build command. This is used in the follow-on
Ie0a165cc6ffc765c03457691901a1dd41ce99d5a.
While we are here, remove the build-gerrit.sh script that isn't used
any more, along with the step that installs it.
Also, refactor the tasks to use include_role (this is also used in the
follow on).
Change-Id: I4f3908e75cbbb7673135a2717f9e51f099a4860e