torture: Prevent jitter from delaying build-only runs

Currently, if the --jitter flag specifies jitter for a --build-only
run, the system will obediently build a kernel, refuse to launch it,
launch the requested number of jitter processes, and wait for the
specified kernel run time, which defaults to 30 minutes.  This is
of course quite pointless.

This commit therefore disables jitter on build-only runs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>