vendor: update buildkit to master@31c870e82a48

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

vendor/golang.org/x/oauth2/internal/token.go

@@ -19,8 +19,6 @@ import (
 	"strings"
 	"sync"
 	"time"
-
-	"golang.org/x/net/context/ctxhttp"
 )
 
 // Token represents the credentials used to authorize
@@ -229,7 +227,7 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
 }
 
 func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
-	r, err := ctxhttp.Do(ctx, ContextClient(ctx), req)
+	r, err := ContextClient(ctx).Do(req.WithContext(ctx))
 	if err != nil {
 		return nil, err
 	}