build: validate build args and labels

[crazy-max]

fixes #2022

Adds validation for build arguments and labels that must have a key to be consistent with previous implementation of the build command in docker/cli.

We already have this validation for bake when we set new overrides:

buildx/bake/bake.go

Lines 430 to 432 in f6b7a3c

	if len(parts) != 2 && keys[1] != "args" {
	return nil, errors.Errorf("invalid override %s, expected target.name=value", v)
	}

buildx/bake/bake.go

Lines 456 to 458 in f6b7a3c

	if len(keys) != 3 {
	return nil, errors.Errorf("invalid key %s, args requires name", parts[0])
	}

but missing for labels.