Replace the always omitted `CPU` environment variable with `numberOfProcessors`. (#161392)

I am not sure if this is a 1:1 comparison, so please advise.

Towards https://github.com/flutter/flutter/issues/161387.

dev/bots/utils.dart

diff --git a/dev/bots/utils.dart b/dev/bots/utils.dart
index c14e015..4f573cc 100644
--- a/dev/bots/utils.dart
+++ b/dev/bots/utils.dart
@@ -426,20 +426,10 @@
   List<String>? tags,
   bool runSkipped = false,
 }) async {
-  int? cpus;
-  final String? cpuVariable = Platform.environment['CPU']; // CPU is set in cirrus.yml
-  if (cpuVariable != null) {
-    cpus = int.tryParse(cpuVariable, radix: 10);
-    if (cpus == null) {
-      foundError(<String>[
-        '${red}The CPU environment variable, if set, must be set to the integer number of available cores.$reset',
-        'Actual value: "$cpuVariable"',
-      ]);
-      return;
-    }
-  } else {
-    cpus = 2; // Don't default to 1, otherwise we won't catch race conditions.
-  }
+  // TODO(matanlurey): Consider Platform.numberOfProcessors instead.
+  // See https://github.com/flutter/flutter/issues/161399.
+  int cpus = 2;
+
   // Integration tests that depend on external processes like chrome
   // can get stuck if there are multiple instances running at once.
   if (forceSingleCore) {