benchmark: Fix mmtests/hackbench

By default hackbench builds v5.14 of the kernel for allmodconfig. But
this has an issue where with a modern toolchain, warnings are raised
causing non-zero exit code from make, which causes hackbench to declare
itself a fail. I didn't notice this before because I've been running in
my VM which didn't have enough disk space to do an allmodconfig build,
so I was only doing defconfig, which doesn't build the code that emits
the warnings.

Fix this problem by overriding the version to v6.12, which does not emit
any warnings. But v6.12 needs cpio so add that to the image. For good
measure also add fakeroot and rsync, which Shrinkwrap found to be needed
for some kernel builds.

Fixes: ea21a40d ("container: Add mmtests benchmark suite")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>