Add top level archs key

[eddiezane]

We should add a top level archs: []string key similar to apko to avoid having to pass a flag.

[kaniini]

I don't think that this is something we should add to Melange. In contrast to apko, we want Melange to be opportunistic when building packages, constrained by the .package.target-architecture property.

Basically, I think archs does the wrong thing here, because it would imply that the melange .yaml files would need to be updated to allow Wolfi to be built on a new architecture, which seems like it would be a lot of work.

I assume the issue is relating to Melange wanting to do multi-arch builds by default? We can and should change that to --arch host.