torture: Make kvm-recheck.sh skip kcsan.sum for build-only runs

Currently, kvm-recheck.sh attempts to create a kcsan.sum file even for
build-only runs.  This results in false-positive bash errors due to
there being no console.log files in that case.  This commit therefore
makes kvm-recheck.sh skip creating the kcsan.sum file for build-only runs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>