output: Make WebAppManifestFormat NotAlternative=true

Updates #8624

output/outputFormat.go

@@ -148,6 +148,7 @@ var (
 		MediaType:      media.WebAppManifestType,
 		BaseName:       "manifest",
 		IsPlainText:    true,
+		NotAlternative: true,
 		Rel:            "manifest",
 	}