exporter: unlazy references in parallel

[jedevc]

From slack:

jed: for the unlazying stuff we do in the exporter - should we perform this in parallel over all the refs? I notice when it's pulling images, we actually pull two different architectures in sequence, instead of in parallel.
It's a pretty quick fix with an errgroup, but maybe it's intentional as is?

tonistiigi: No reason this could be intentional

We can unlazy all the references in parallel, since these perform network operations, we should attempt to pull them all together, instead of one-after-each-other.

We can also use the EachRef helper to simplify the iteration over all the references - I think this now would include the attestation references? I don't think this affects anything since attestation references weren't lazily computed, and it just makes sure that they are included in the multi-provider.