Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/isolate/example.expected/none.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 38 matching lines @@
       "--init",
       "--recursive"
     ],
     "cwd": "[SLAVE_BUILD]/swarming.client",
     "name": "submodule update (swarming_client)"
   },
   {
     "cmd": [
       "cat"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "read test spec",
     "stdout": "/path/to/tmp/json",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@null@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[isolate]/resources/find_isolated_tests.py",
       "--build-dir",
       "[BUILD]",
       "--output-json",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "find isolated tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_WARNINGS@@@",
       "@@@SET_BUILD_PROPERTY@swarm_hashes@{}@@@"
     ]
   }
 ]