bump agnhost to 2.20

Author: BenTheElder

test/utils/image/manifest.go

@@ -202,7 +202,7 @@ const (
 
 func initImageConfigs() map[int]Config {
 	configs := map[int]Config{}
-	configs[Agnhost] = Config{promoterE2eRegistry, "agnhost", "2.19"}
+	configs[Agnhost] = Config{promoterE2eRegistry, "agnhost", "2.20"}
 	configs[AgnhostPrivate] = Config{PrivateRegistry, "agnhost", "2.6"}
 	configs[AuthenticatedAlpine] = Config{gcAuthenticatedRegistry, "alpine", "3.7"}
 	configs[AuthenticatedWindowsNanoServer] = Config{gcAuthenticatedRegistry, "windows-nanoserver", "v1"}