Update gomod and vendor

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

@@ -0,0 +1,23 @@
+// +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()
+}