Enable multiple exporters

[jedevc]

Follow-up to moby/buildkit#4134 - with this merged, we should be able to remove the explicit checks that prevent using multiple exporters.

However, there's still the question of how this should interact with --metadata-file - see moby/buildkit#4532 for more information. We could:

1. ignore this issue
2. block adding this functionality until resolving that upstream
3. don't allow users to use --metadata-file with multiple exporters until resolving that upstream (but note that the most recent buildx release has this issue with multiple cache exporters today)