Fixup attestation platforms for the local exporter

[jedevc]

⬆️ Follow-up to #3197.

This PR corrects the behaviour of the local exporter with the multi-platform argument. Essentially, the problem boils down to the fact that we can't distinguish at the exporter level between the results produced by the following:

$ buildctl build --opt platform=linux/amd64 --opt attest:sbom
$ buildctl build --opt platform=linux/amd64 --opt attest:sbom --opt build-arg:BUILDKIT_MULTI_PLATFORM=true

We'd like the former to produce a flat file structure through the local exporter, and the latter to produce an explicitly nested file structure through the same exporter. However, both of these produce Refs instead of a singular Ref, so we can't just look at the Result to know which one.

To solve this, we can handle this case like SOURCE_DATE_EPOCH, and detect in the control API boundary if the multi-platform option is set and directly forward it to the exporter, which we can use to make the right call about what format of output to produce. As part of this, we fixup the client to send all frontend attributes to the main solve request for Solves inside Builds - since we would now be having to filter through all attestation arguments, SOURCE_DATE_EPOCH and BUILDKIT_MULTI_PLATFORM/multi-platform - which is frustrating and limits upgrade paths. This behavior was previously broken on master, the fallback for SOURCE_DATE_EPOCH in the control API boundary would never be activated by buildctl or buildx, since they use Solves inside Builds exclusively.

We should probably pick this into the release, since without it, BUILDKIT_MULTI_PLATFORM won't work as intended for the local exporter. Additionally, the fallback for SOURCE_DATE_EPOCH won't work correctly.