Filter `get_oci_metadata` by architecture

[mxmxchere]

I am not 100% sure whether the current behaviour actually is a bug or serves a purpose, but it might well be just a bug:

The function get_oci_metadata is meant to return a list of files/layers (artifacts) that should be added to a manifest. For amd64 and arm64 it goes through the build directory and the info.yamls to compose this information. Instead it should have the architecture as parameter and return only the information relevant for amd64 or arm64 (or ...).

[mxmxchere]

I just talked with @5kt and he also assumes that the filtering via the architecture is necessary. I will change it according to that, if we find out later that the current approach is better we can still revert