Update go.mod and vendor

vendor/github.com/moby/buildkit/source/git/gitsource_unix.go

@@ -1,35 +0,0 @@
-// +build !windows
-
-package git
-
-import (
-	"context"
-	"os/exec"
-	"syscall"
-	"time"
-)
-
-func runProcessGroup(ctx context.Context, cmd *exec.Cmd) error {
-	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
-	if err := cmd.Start(); err != nil {
-		return err
-	}
-	waitDone := make(chan struct{})
-	go func() {
-		select {
-		case <-ctx.Done():
-			syscall.Kill(-cmd.Process.Pid, syscall.SIGTERM)
-			go func() {
-				select {
-				case <-waitDone:
-				case <-time.After(10 * time.Second):
-					syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
-				}
-			}()
-		case <-waitDone:
-		}
-	}()
-	err := cmd.Wait()
-	close(waitDone)
-	return err
-}