torture: Make "--dryrun script" use same environment as normal run

In a normal torture-test run, the script inherits its environment
variables, but this does not work when producing a script that is
to run later.  Therefore, definitions and exports are prepended to
a dryrun script but not to a script that is run immediately.  This
commit reconciles this by placing definitions and exports at the
beginning of the script in both cases.

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

Conflicts:
	tools/testing/selftests/rcutorture/bin/kvm.sh