Add a flag to defer errors occurring in matrix build targets

[korbin]

Description

I have a large matrix build emitting a number of products from multiple sequential 5+ hour build stages.

If one of these matrix targets fails for whatever reason, I'd like the others to run to completion or failure.

Currently, the entire build process dies upon the first error encountered in any individual matrix target, throwing away a bunch of valuable work for other running matrix targets.

I'd like a flag, docker buildx bake --defer-exit (or whatever) that enables this behavior.

[tonistiigi]

We should somehow combine this with functionality of #1197 . Smth. like --strategy=parallel|sync-output|defer-error maybe