refactor: execute the predeploy script during the pre-release-builder plugin trigger

This fixes an issue where the post-release-builder trigger inadvertently modifies the image. We take advantage of the fact that env vars are available at the `pre-release-builder` phase to ensure any image modifications occur as appropriate.

plugins/app-json/src/triggers/triggers.go

@@ -61,6 +61,11 @@ func main() {
 		appName := flag.Arg(1)
 		image := flag.Arg(2)
 		err = appjson.TriggerPostReleaseBuilder(builderType, appName, image)
+	case "pre-release-builder":
+		builderType := flag.Arg(0)
+		appName := flag.Arg(1)
+		image := flag.Arg(2)
+		err = appjson.TriggerPreReleaseBuilder(builderType, appName, image)
 	case "report":
 		appName := flag.Arg(0)
 		err = appjson.ReportSingleApp(appName, "", "")