Allow for anchor net in regression testing.

To allow anchor net to be avaiable to test against similar to the code in lczero-server/main.go here:

[...]
	// Regression tests for current best.  Done here because the 'end of match' code logic isn't thread safe, so could create matches multiple times.
	var prevNetwork1 db.Network
	err = db.GetDB().Where("network_number = ?", bestNetwork.NetworkNumber-3).First(&prevNetwork1).Error
	if err == nil {
		createMatch(trainingRun, &prevNetwork1, true, string(params[:]))
	}
	var prevNetwork2 db.Network
	err = db.GetDB().Where("network_number = ?", bestNetwork.NetworkNumber-10).First(&prevNetwork2).Error
	if err == nil {
		createMatch(trainingRun, &prevNetwork2, true, string(params[:]))
	}
[...]

the desired network would need to be available. The following code in lczero-client/lc0_main.go deletes old networks:

[...]
func removeAllExcept(dir string, sha string, keepTime string) error {
	files, err := ioutil.ReadDir(dir)
	if err != nil {
		return err
	}
	for _, file := range files {
		if file.Name() == sha {
			continue
		}
		timeLimit, _ := time.ParseDuration(keepTime)
		if time.Since(file.ModTime()) < timeLimit {
			continue
		}
		fmt.Printf("Removing %v\n", file.Name())
		err := os.RemoveAll(filepath.Join(dir, file.Name()))
		if err != nil {
			return err
		}
	}
	return nil
}
[...]

However, it should be relatively simple to include a condition to not delete specific networks. E.g.

		if file.Name() == "5c222ccd1ccbed2666b3a8ef94d8833d386d2168d51c3e99a0a3fc37a56d2569" {
			continue
		}

It would probably be better to store the sha in a variable declaration or a configuration file.

Start in lc0_main.go at removeAllExcept and trace its callers and network-retention configuration. Decide how the specified anchor network should be identified and preserved without disrupting existing cleanup. Done means the anchor network remains available for regression testing while other eligible old networks are still removed.