Enable post-compile test isolation for all CI builders.

scripts/slave/recipes/chromium.expected/arm.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.fyi",
       "--builder",
       "Linux ARM Cross-Compile",
@@ skipping 61 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
       "GYP_CROSSCOMPILE": "1",
-      "GYP_DEFINES": "arm_float_abi=hard component=static_library target_arch=arm test_isolation_mode=archive test_isolation_outdir=https://isolateserver.appspot.com"
+      "GYP_DEFINES": "arm_float_abi=hard component=static_library target_arch=arm test_isolation_mode=prepare"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/src/testing/buildbot/chromium.fyi.json",
       "/path/to/tmp/json"
@@ skipping 76 matching lines @@
       "--json",
       "/path/to/tmp/json"
     ],
     "name": "checkdeps",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@[]@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   }
 ]