fix(@schematics/angular): silently skip when the build target already uses one of the new builders

Author: Deku-nattsu

packages/schematics/angular/migrations/use-application-builder/migration.ts

@@ -178,6 +178,11 @@ function updateProjects(tree: Tree, context: SchematicContext) {
         continue;
       }
 
+      if (buildTarget.builder.startsWith('@angular/build')) {
+        // Already using @angular/build builders
+        continue;
+      }
+
       if (
         buildTarget.builder !== Builders.BrowserEsbuild &&
         buildTarget.builder !== Builders.Browser