Update go.mod and vendor

vendor/github.com/moby/buildkit/util/appcontext/appcontext.go

@@ -1,41 +0,0 @@
-package appcontext
-
-import (
-	"context"
-	"os"
-	"os/signal"
-	"sync"
-
-	"github.com/sirupsen/logrus"
-)
-
-var appContextCache context.Context
-var appContextOnce sync.Once
-
-// Context returns a static context that reacts to termination signals of the
-// running process. Useful in CLI tools.
-func Context() context.Context {
-	appContextOnce.Do(func() {
-		signals := make(chan os.Signal, 2048)
-		signal.Notify(signals, terminationSignals...)
-
-		const exitLimit = 3
-		retries := 0
-
-		ctx, cancel := context.WithCancel(context.Background())
-		appContextCache = ctx
-
-		go func() {
-			for {
-				<-signals
-				cancel()
-				retries++
-				if retries >= exitLimit {
-					logrus.Errorf("got %d SIGTERM/SIGINTs, forcing shutdown", retries)
-					os.Exit(1)
-				}
-			}
-		}()
-	})
-	return appContextCache
-}