build: don't generate local state for subrequests

[crazy-max]

Relates to https://github.com/docker/docs/actions/runs/11502549192/job/32017891983#step:6:21

Warning: Failed to export build record: /home/runner/work/_temp/docker-actions-toolkit-16m5IG/export/rec.dockerbuild not found

When generating build summary in our GitHub Actions, we are using the buildx.build.ref from metadata to export the build record. When using call, this attribute is not set:

buildx/build/build.go

Lines 525 to 526 in 746eadd

	if opt.CallFunc == nil {
	rr.ExporterResponse["buildx.build.ref"] = buildRef

Which is the right behavior as no build record is written for subrequests anyway.

But we also have a fallback in our GitHub Actions in case buildx.build.ref is not set that checks if a local state is available as part of this build. And in this case we have one as we are generating a local state for subrequests but I don't think it makes sense to do so.

cc @dvdksn