vendor: update buildkit to v0.16.0-rc2

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/google.golang.org/grpc/metadata/metadata.go

@@ -25,8 +25,14 @@ import (
 	"context"
 	"fmt"
 	"strings"
+
+	"google.golang.org/grpc/internal"
 )
 
+func init() {
+	internal.FromOutgoingContextRaw = fromOutgoingContextRaw
+}
+
 // DecodeKeyValue returns k, v, nil.
 //
 // Deprecated: use k and v directly instead.
@@ -238,16 +244,13 @@ func copyOf(v []string) []string {
 	return vals
 }
 
-// FromOutgoingContextRaw returns the un-merged, intermediary contents of rawMD.
+// fromOutgoingContextRaw returns the un-merged, intermediary contents of rawMD.
 //
 // Remember to perform strings.ToLower on the keys, for both the returned MD (MD
 // is a map, there's no guarantee it's created using our helper functions) and
 // the extra kv pairs (AppendToOutgoingContext doesn't turn them into
 // lowercase).
-//
-// This is intended for gRPC-internal use ONLY. Users should use
-// FromOutgoingContext instead.
-func FromOutgoingContextRaw(ctx context.Context) (MD, [][]string, bool) {
+func fromOutgoingContextRaw(ctx context.Context) (MD, [][]string, bool) {
 	raw, ok := ctx.Value(mdOutgoingKey{}).(rawMD)
 	if !ok {
 		return nil, nil, false