Minor but crucial fix to the command run for ios-wpr-simulator.

I accidentally modify cmd (which is unused, but passed in due to inherited function definition),
instead of recipe_cmd (which is used, and generated by this function). This is not caught by tests
as we mock subprocess.

Bug: 892381
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I8960fce63b98f68d2cf65792d4bd6e8bc971dcd0
Reviewed-on: https://chromium-review.googlesource.com/c/1287196Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Commit-Queue: ericale <ericale@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600610}

ios/build/bots/scripts/test_runner.py

@@ -1106,9 +1106,9 @@ class WprProxySimulatorTestRunner(SimulatorTestRunner):
           for test_arg in self.test_args:
             recipe_cmd.extend(['-c', test_arg])
 
-          cmd.append(self.app_path)
+          recipe_cmd.append(self.app_path)
           if self.xctest_path:
-            cmd.append(self.xctest_path)
+            recipe_cmd.append(self.xctest_path)
 
           proc = subprocess.Popen(
               recipe_cmd,