Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/chromium_tests/example.expected/basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.testing.master",
       "--builder",
       "TestBuilder",
@@ skipping 10 matching lines @@
       "--patchset",
       "1",
       "--rietveld_server",
       "https://codereview.chromium.org",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--force"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_root\": \"src\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_nacl_revision\": \"d998e125e8253980d76e269b7982aeeefc1b9b50\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_nacl_revision_cp\": \"refs/heads/master@{#274981}\", @@@",
@@ skipping 42 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/src/testing/buildbot/foo.json",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "read filter exclusion spec",
     "~followup_annotations": [
       "@@@STEP_TEXT@path: 'testing/buildbot/foo.json'@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"exclusions\": [@@@",
       "@@@STEP_LOG_LINE@json.output@      \"f.*\"@@@",
       "@@@STEP_LOG_LINE@json.output@    ]@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"chromium\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"exclusions\": []@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(0)"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "analyze",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@excluded_files@foo.cc (regex = 'f.*')@@@",
       "@@@STEP_LOG_END@excluded_files@@@",
       "@@@STEP_LOG_LINE@analyze_details@{@@@",
       "@@@STEP_LOG_LINE@analyze_details@  \"compile_targets\": [@@@",
       "@@@STEP_LOG_LINE@analyze_details@    \"base_unittests\", @@@",
       "@@@STEP_LOG_LINE@analyze_details@    \"all\"@@@",
       "@@@STEP_LOG_LINE@analyze_details@  ], @@@",
       "@@@STEP_LOG_LINE@analyze_details@  \"original_compile_targets\": [@@@",
       "@@@STEP_LOG_LINE@analyze_details@    \"all\"@@@",
       "@@@STEP_LOG_LINE@analyze_details@  ], @@@",
       "@@@STEP_LOG_LINE@analyze_details@  \"original_exes\": [@@@",
       "@@@STEP_LOG_LINE@analyze_details@    \"base_unittests\"@@@",
       "@@@STEP_LOG_LINE@analyze_details@  ], @@@",
       "@@@STEP_LOG_LINE@analyze_details@  \"self.m.filter.compile_targets\": [@@@",
       "@@@STEP_LOG_LINE@analyze_details@    \"all\"@@@",
       "@@@STEP_LOG_LINE@analyze_details@  ], @@@",
       "@@@STEP_LOG_LINE@analyze_details@  \"self.m.filter.matching_exes\": [@@@",
       "@@@STEP_LOG_LINE@analyze_details@    \"base_unittests\"@@@",
       "@@@STEP_LOG_LINE@analyze_details@  ]@@@",
       "@@@STEP_LOG_LINE@analyze_details@}@@@",
       "@@@STEP_LOG_END@analyze_details@@@"
     ]
   }
 ]