Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/trigger/example.expected/buildbot_changes.json

 [
   {
     "cmd": [],
+    "cwd": "[SLAVE_BUILD]",
     "name": "trigger",
     "trigger_specs": [
       {
         "buildbot_changes": [
           {
             "author": "someone@chromium.org",
             "comments": "hello world!",
             "revision": "deadbeef"
           }
         ],
         "builder_name": "cross-compiler"
       }
     ],
     "~followup_annotations": [
       "@@@STEP_TEXT@cross-compiler@@@"
     ]
   }
 ]