Go back to `-j1` in flutter_tools tests (#24196)

To see if it stops the Cirrus crashing

dev/bots/test.dart

@@ -4,7 +4,6 @@
 
 import 'dart:async';
 import 'dart:io';
-import 'dart:math' as math;
 
 import 'package:path/path.dart' as path;
 
@@ -230,9 +229,7 @@ Future<void> _pubRunTest(
   String testPath,
   bool enableFlutterToolAsserts = false
 }) {
-  final List<String> args = <String>['run', 'test', '-rcompact'];
-  final int concurrency = math.max(1, Platform.numberOfProcessors - 1);
-  args.add('-j$concurrency');
+  final List<String> args = <String>['run', 'test', '-rcompact', '-j1'];
   if (!hasColor)
     args.add('--no-color');
   if (testPath != null)