vendor: update buildkit

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

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

@@ -123,6 +123,13 @@ func (md MD) Append(k string, vals ...string) {
 	md[k] = append(md[k], vals...)
 }
 
+// Delete removes the values for a given key k which is converted to lowercase
+// before removing it from md.
+func (md MD) Delete(k string) {
+	k = strings.ToLower(k)
+	delete(md, k)
+}
+
 // Join joins any number of mds into a single MD.
 //
 // The order of values for each key is determined by the order in which the mds