To test current buildx we need a later skopeo that understands the OCI
manifest formats. c.f. the zuul-executor image updates with
Iab667a92a5b6e6f8591db2aa435a782913d9d34f.
A recent enough version (1.9) isn't packaged for any LTS distros. So
add an upstream option, but it's only implemented (for now) on Jammy,
where we will test with it.
Change-Id: I206a3cbfb16575f409771d96c2b7e49929e61a49
32be2d9067