Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/v8/example.expected/forced_build.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "Fake_Master",
       "--builder",
       "Fake Builder",
       "--slave",
       "TestSlavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'chromium_trunk': 'svn://svn-mirror.golo.chromium.org/chrome/trunk'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'v8', 'url': 'https://chromium.googlesource.com/v8/v8'}]",
       "--root",
       "v8",
       "--revision_mapping_file",
       "{\"v8\": \"got_revision\"}",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "v8@HEAD",
       "--force",
       "--no_shallow"
     ],
+    "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\": \"v8\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision\": \"f2cbea7e89a45f46819777bb78a1f5bd0241f64c\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision_cp\": \"refs/heads/master@{#50110}\"@@@",
@@ skipping 41 matching lines @@
     "~followup_annotations": [
       "@@@STEP_LINK@Results Dashboard@https://chrome-perf.googleplex.com/report?rev=12345&masters=Fake_Master&tests=v8%2FExample2&bots=ia32@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[perf_dashboard]/resources/post_json.py"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "perf dashboard post",
     "stdin": "{\"data\": [{\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"8.16496580928\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/Richards\", \"units\": \"score\", \"value\": \"20.0\"}, {\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"0.0\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/DeltaBlue\", \"units\": \"ms\", \"value\": \"1.2\"}, {\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"8.16496580928\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/Richards\", \"units\": \"score\", \"value\": \"20.0\"}, {\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"0.0\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/DeltaBlue\", \"units\": \"ms\", \"value\": \"1.2\"}], \"url\": \"https://chrome-perf.googleplex.com/add_point\"}"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[v8]/resources/merge_perf_results.py",
       "[SLAVE_BUILD]/test_output1.json",
       "[SLAVE_BUILD]/test_output2.json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "merge perf results",
     "stdout": "/path/to/tmp/json",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"res\": \"the result\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "echo",
       "the result"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "do something with the results"
   }
 ]