FileAction Rm doesn't seem to respect AllowWildcard

[hinshun]

Attempting to remove files based on wildcard doesn't seem to work.

Reading the code, it also looks like it doesn't respect wildcards like copy does:

buildkit/solver/llbsolver/file/backend.go

Lines 102 to 116 in 50a406a

	func rm(ctx context.Context, d string, action pb.FileActionRm) error {
	p, err := fs.RootPath(d, filepath.Join(filepath.Join("/", action.Path)))
	if err != nil {
	return err
	}
	if err := os.RemoveAll(p); err != nil {
	if os.IsNotExist(errors.Cause(err)) && action.AllowNotFound {
	return nil
	}
	return err
	}
	return nil
	}