torture: Create qemu-cmd in --buildonly runs

One reason to do a --buildonly run is to use the build products elsewhere,
for example, to do the actual test on some other system.  Part of doing
the test is the actual qemu command, which is not currently produced
by --buildonly runs.  This commit therefore causes --buildonly runs to
create this file.

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