Recipes: make cwd default to slave_build

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[gitiles]/resources/gerrit_client.py",
       "--json-file",
       "/path/to/tmp/json",
       "--url",
       "https://chromium.googlesource.com/chromiumos/tryjobs.git/+/a94a8fe5ccb19ba61c4c0873d391e987982fbbd3?format=json",
       "--attempts",
       "10"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "Fetch tryjob commit",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"author\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"email\": \"testauthor@fake.chromium.org\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"name\": \"Test Author\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"time\": \"Mon Jan 01 00:00:00 2015\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"commit\": \"a94a8fe5ccb19ba61c4c0873d391e987982fbbd3\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"committer\": {@@@",
@@ skipping 24 matching lines @@
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/tools/pycurl.py",
       "https://chromium.googlesource.com/chromiumos/tryjobs.git/+/a94a8fe5ccb19ba61c4c0873d391e987982fbbd3/user/user.12345?format=text",
       "--outfile",
       "/path/to/tmp/",
       "--attempts",
       "10"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "Fetch tryjob descriptor (user/user.12345)",
     "~followup_annotations": [
       "@@@STEP_TEXT@extra_args: [u'--timeout', u'14400', u'--remote-trybot', u'--remote-version=4']<br/>name: 12345<br/>email: testauthor@fake.chromium.org@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromiumos.tryserver",
       "--builder",
       "etc",
       "--slave",
       "test",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'chromite', 'url': 'https://chromium.googlesource.com/chromiumos/chromite.git'}]",
       "--root",
       "chromite",
       "--revision_mapping_file",
       "{}",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "chromite@master:",
       "--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\": \"chromite\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {}, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"chromite\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
@@ skipping 12 matching lines @@
       "--clobber",
       "--timeout",
       "14400",
       "--remote-trybot",
       "--remote-version=4",
       "etc"
     ],
     "name": "cbuildbot [etc]"
   }
 ]