Teach `build-chunked-oci` to stop grouping RPMs from different SRPMs

[jlebon]

According to @cgwalters, the last blocker to be able to have images with many layers is containers/skopeo@a31470d. Once we can rely on that, we should be able to make chunking even more granular, which would then obviate the need for #5459 I think, at least for FCOS.

Hmm, possibly the existing --max-layers is sufficient for this. We just need to verify that putting it to a number sufficiently high gives us the behaviour we want (i.e. that grouping happens per-SRPM). If not, then possibly we need a new switch for that behaviour.