[web] Update builder json generator to reflect recent changes (#55307)

Recent [changes](#54584) affected our builder configs, but the script that generates those builder configs wasn't updated.

Author: mdebbar

lib/web_ui/dev/generate_builder_json.dart

@@ -30,6 +30,7 @@ Map<String, dynamic> _getArtifactBuildStep() {
     ],
     'gclient_variables': <String, dynamic>{
       'download_android_deps': false,
+      'download_jdk': false,
       'download_emsdk': true,
     },
     'gn': <String>[
@@ -141,6 +142,7 @@ Iterable<dynamic> _getTestStepsForPlatform(
         ],
         'gclient_variables': <String, dynamic>{
           'download_android_deps': false,
+          'download_jdk': false,
         },
         'dependencies': <String>[
           'web_tests/artifacts',

lib/web_ui/dev/test_runner.dart

@@ -339,7 +339,7 @@ class TestCommand extends Command<bool> with ArgUtils<bool> {
         'builders',
         'linux_web_engine.json',
       ));
-      configFile.writeAsStringSync(configString);
+      configFile.writeAsStringSync('$configString\n');
       return true;
     }
     if (isList || isVerbose) {