vendor: update buildkit to master@cbfd4023383d

Signed-off-by: Justin Chadwell <me@jedevc.com>

vendor/github.com/moby/buildkit/util/contentutil/storewithprovider.go

@@ -0,0 +1,24 @@
+package contentutil
+
+import (
+	"context"
+
+	"github.com/containerd/containerd/content"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+func NewStoreWithProvider(cs content.Store, p content.Provider) content.Store {
+	return &storeWithProvider{Store: cs, p: p}
+}
+
+type storeWithProvider struct {
+	content.Store
+	p content.Provider
+}
+
+func (cs *storeWithProvider) ReaderAt(ctx context.Context, desc ocispecs.Descriptor) (content.ReaderAt, error) {
+	if ra, err := cs.p.ReaderAt(ctx, desc); err == nil {
+		return ra, nil
+	}
+	return cs.Store.ReaderAt(ctx, desc)
+}