Recipes: make cwd default to slave_build

scripts/slave/recipes/infra/infra_continuous.expected/infra_internal.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.testing.master",
       "--builder",
       "infra-internal-continuous",
@@ skipping 10 matching lines @@
       "--patchset",
       "1",
       "--rietveld_server",
       "https://codereview.chromium.org",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "infra_internal@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\": \"infra_internal\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision\": \"0a18b32e3a93913e1090d1ee6b8965c6dc3116d6\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision_cp\": \"refs/heads/master@{#190894}\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"infra_internal\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_revision@\"0a18b32e3a93913e1090d1ee6b8965c6dc3116d6\"@@@",
       "@@@SET_BUILD_PROPERTY@got_revision_cp@\"refs/heads/master@{#190894}\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "test.py",
       "test"
     ],
     "cwd": "[SLAVE_BUILD]/infra_internal",
     "name": "infra python tests"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/infra_internal/go/env.py",
       "python",
       "[SLAVE_BUILD]/infra_internal/go/test.py"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "infra go tests"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/infra_internal/build/build.py",
       "--upload",
       "--service-account-json",
       "/creds/service_accounts/service-account-cipd-builder.json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "build cipd packages"
   }
 ]