tests/docker: pin powerpc-user-cross to a snapshot

tests/docker: pin powerpc-user-cross to a snapshot

Now Jessie has entered LTS the powerpc architecture has been dropped
so we can no longer build the image from scratch. However we can use
the snapshot archive to build the last working version.

This now only lives on an example of setting up a user-cross image as
at least on x86-64 we can use the Buster packaged cross compiler for
building test images.

Suggested-by: Philippe Mathieu-Daudé <[email protected]>
Cc: David Gibson <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>