Use pub inside the Flutter directory (#57704)

dev/devicelab/lib/framework/utils.dart

@@ -437,6 +437,9 @@ Future<String> evalFlutter(String command, {
 String get dartBin =>
     path.join(flutterDirectory.path, 'bin', 'cache', 'dart-sdk', 'bin', 'dart');
 
+String get pubBin =>
+    path.join(flutterDirectory.path, 'bin', 'cache', 'dart-sdk', 'bin', 'pub');
+
 Future<int> dart(List<String> args) => exec(dartBin, args);
 
 /// Returns a future that completes with a path suitable for JAVA_HOME

dev/devicelab/lib/tasks/perf_tests.dart

@@ -780,14 +780,14 @@ class DevToolsMemoryTest {
   }
 
   Future<void> _launchDevTools() async {
-    await exec('pub', <String>[
+    await exec(pubBin, <String>[
       'global',
       'activate',
       'devtools',
       '0.2.5',
     ]);
     _devToolsProcess = await startProcess(
-      'pub',
+      pubBin,
       <String>[
         'global',
         'run',