fix: keep the digest of an image on fly m create

[kzys]

Change Summary

This change fixes fly pg create too.

What and Why:

How:

Related to:

---

Documentation

• Fresh Produce
• In superfly/docs, or asked for help from docs team
• n/a

[davissp14]

This shouldn't actually impact fly pg create, but good stuff none the less!

PG has it's own special logic:

flyctl/flypg/launcher.go

Lines 100 to 112 in 74f80ea

	if config.ImageRef == "" {
	imageRepo := "flyio/postgres"
	if config.Manager == ReplicationManager {
	imageRepo = "flyio/postgres-flex"
	}
	imageRef, err := client.GetLatestImageTag(ctx, imageRepo, config.SnapshotID)
	if err != nil {
	return err
	}
	config.ImageRef = imageRef
	}