build: set same ref when building on multiple nodes

[crazy-max]

Currently when building with bake we are setting the build ref for each target which results with the same ref being set in solve request when building on multiple nodes:

buildx/commands/bake.go

Lines 232 to 238 in 5656c98

	for k, b := range bo {
	b.Ref = identity.NewID()
	b.GroupRef = groupRef
	b.ProvenanceResponseMode = prm
	refs = append(refs, b.Ref)
	bo[k] = b
	}

But for build we don't do this in commands package and therefore it will generate a different one for each node:

• buildx/build/build.go

  Line 229 in 5656c98

  so, release, err := toSolveOpt(ctx, np.Node(), multiDriver, opt, gatewayOpts, configDir, addVCSLocalDir, w, docker)

• buildx/build/opt.go

  Line 108 in 5656c98

  so.Ref = identity.NewID()

@tonistiigi I think we should align build with bake to have the same ref across nodes but let me know if this should be the other way around.

cc @fiam