Use flutter pub get, not dart pub get, on the autoroller (flutter#169922)

Fixing
https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8713094164676167457/+/u/run_roll-packages_script/stdout

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Author: mosuem

dev/conductor/core/lib/src/packages_autoroller.dart

@@ -154,7 +154,7 @@ This PR was generated by the automated
   ) async {
     final String entrypoint = '${repoRoot.path}/dev/tools/bin/generate_gradle_lockfiles.dart';
     final File target = tempDir.childFile('generate_gradle_lockfiles');
-    await framework.streamDart(<String>[
+    await framework.streamFlutter(<String>[
       'pub',
       'get',
     ], workingDirectory: '${repoRoot.path}/dev/tools');

dev/conductor/core/lib/src/repository.dart

@@ -594,13 +594,14 @@ class FrameworkRepository extends Repository {
     List<String> args, {
     void Function(String)? stdoutCallback,
     void Function(String)? stderrCallback,
+    String? workingDirectory,
   }) async {
-    final String workingDirectory = (await checkoutDirectory).path;
+    final String repoWorkingDirectory = (await checkoutDirectory).path;
 
     return _streamProcess(<String>[
-      fileSystem.path.join(workingDirectory, 'bin', 'flutter'),
+      fileSystem.path.join(repoWorkingDirectory, 'bin', 'flutter'),
       ...args,
-    ], workingDirectory: workingDirectory);
+    ], workingDirectory: workingDirectory ?? repoWorkingDirectory);
   }
 
   Future<io.Process> _streamProcess(

dev/conductor/core/test/packages_autoroller_test.dart

@@ -252,7 +252,7 @@ void main() {
       const FakeCommand(command: <String>['git', 'rev-parse', 'HEAD'], stdout: 'deadbeef'),
       const FakeCommand(
         command: <String>[
-          '$checkoutsParentDirectory/flutter_conductor_checkouts/framework/bin/dart',
+          '$checkoutsParentDirectory/flutter_conductor_checkouts/framework/bin/flutter',
           'pub',
           'get',
         ],
@@ -344,7 +344,7 @@ void main() {
       const FakeCommand(command: <String>['git', 'rev-parse', 'HEAD'], stdout: 'deadbeef'),
       const FakeCommand(
         command: <String>[
-          '$checkoutsParentDirectory/flutter_conductor_checkouts/framework/bin/dart',
+          '$checkoutsParentDirectory/flutter_conductor_checkouts/framework/bin/flutter',
           'pub',
           'get',
         ],