use --arch for ops pkg push cmd (with sanitisation for amd64)

[radiosilence]

Basically this completes the loop for pushing packages with multiple arches via CLI.

Seeing as the current --arch flag accepts either amd64 (as opposed to x86_64) or arm64, I'm manually changing this in the request data when it's pushed to the repo.

Also falls back to x86_64 if it's blank/unset

Also it slightly changes the behaviour when searching local packages, as previously it was erroring out if one of the existing arches didn't exist yet, so we just log it and continue instead.