torturescript: Don't rerun failed rcutorture builds

If the build fails when running multiple instances of a given rcutorture
scenario, for example, using the kvm.sh --configs "8*RUDE01" argument,
the build will be rerun an additional seven times.  This is in some sense
correct, but it can waste significant time.  This commit therefore checks
for a prior failed build and simply copies over that build's output.

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