make `cosa diff` support passing in the architecture

[dustymabe]

Something like this should work:

cosa buildfetch --arch=ppc64le --build=40.20241017.20.0 --artifact=ostree
cosa buildfetch --arch=ppc64le --build=40.20241019.20.0 --artifact=ostree
cosa diff --arch=ppc64le --from=40.20241017.20.0 --to=40.20241019.20.0 --rpms

but right now cosa diff doesn't take in an --arch argument and definitely doesn't work if you aren't on the architecture you want to get information about.

[dustymabe]

fixed in #3999