Update go.mod and vendor

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

@@ -1,23 +0,0 @@
-// +build windows
-
-package git
-
-import (
-	"context"
-	"os/exec"
-)
-
-func runProcessGroup(ctx context.Context, cmd *exec.Cmd) error {
-	if err := cmd.Start(); err != nil {
-		return err
-	}
-	waitDone := make(chan struct{})
-	go func() {
-		select {
-		case <-ctx.Done():
-			cmd.Process.Kill()
-		case <-waitDone:
-		}
-	}()
-	return cmd.Wait()
-}