CrOS: Use local Chromite checkout for configs.

scripts/slave/recipes/cros/cbuildbot_tryjob.expected/unknown_config.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromiumos.tryserver",
       "--builder",
       "etc",
@@ skipping 29 matching lines @@
   {
     "cmd": [
       "git",
       "checkout",
       "master"
     ],
     "cwd": "[SLAVE_BUILD]/chromite",
     "name": "checkout chromite branch [master]"
   },
   {
+    "cmd": [
+      "python",
+      "-u",
+      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
+      "[SLAVE_BUILD]/chromite/cbuildbot/config_dump.json",
+      "/path/to/tmp/json"
+    ],
+    "cwd": "[SLAVE_BUILD]",
+    "name": "read chromite config"
+  },
+  {
     "allow_subannotations": true,
     "cmd": [
       "[SLAVE_BUILD]/chromite/bin/cbuildbot",
       "--buildroot",
       "[ROOT]/cbuild/etc_master",
       "--clobber",
       "--timeout",
       "14400",
       "--remote-trybot",
       "--remote-version=4",
       "xxx-fakeboard-fakebuild"
     ],
     "name": "cbuildbot [xxx-fakeboard-fakebuild]"
   }
 ]