Recipes: make cwd default to slave_build

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/git_setup.py",
       "--path",
       "[SLAVE_BUILD]/swarming.client",
       "--url",
       "https://chromium.googlesource.com/external/swarming.client.git"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "git setup (swarming_client)"
   },
   {
     "cmd": [
       "git",
       "retry",
       "fetch",
       "origin",
       "master"
     ],
@@ skipping 41 matching lines @@
     "cwd": "[SLAVE_BUILD]/swarming.client",
     "name": "submodule update (swarming_client)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/swarming.client/swarming.py",
       "--version"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "swarming.py --version",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_TEXT@0.4.4@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/swarming.client/isolate.py",
       "--version"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "isolate.py --version",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_TEXT@0.3.1@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(1)"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "swarming.py is too old",
     "~followup_annotations": [
       "@@@STEP_TEXT@Expecting at least v20.0.0, got v0.4.4@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "Expecting at least v20.0.0, got v0.4.4",
     "status_code": 1
   }
 ]