fix: adapt to new unkown version marker

Follows 7a0d18ceb6.

cli/app/rollback.go

@@ -85,8 +85,7 @@ recipes.
 		}
 
 		var availableDowngrades []string
-		if deployedVersion == "" {
-			deployedVersion = "unknown"
+		if deployedVersion == "unknown" {
 			availableDowngrades = versions
 			logrus.Warnf("failed to determine version of deployed %s", app.Name)
 		}