Recipes: make cwd default to slave_build

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_win_x64_gn.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "win_x64_gn",
       "--slave",
       "slavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/external/webrtc'}, {'deps_file': 'DEPS', 'managed': True, 'name': 'webrtc-limited', 'url': 'https://chrome-internal.googlesource.com/chrome/deps/webrtc-limited'}]",
       "--root",
       "src",
       "--revision_mapping_file",
       "{\"src\": \"got_revision\"}",
       "--issue",
       "666666",
       "--patchset",
       "1",
       "--rietveld_server",
       "https://fake.rietveld.url",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@12345"
     ],
+    "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\": \"src\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision\": \"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision_cp\": \"refs/heads/master@{#170242}\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"src\", @@@",
       "@@@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@\"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\"@@@",
       "@@@SET_BUILD_PROPERTY@got_revision_cp@\"refs/heads/master@{#170242}\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[webrtc]\\resources\\cleanup_files.py",
       "[SLAVE_BUILD]\\src\\out"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "clean test output files"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\cleanup_temp.py"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "cleanup_temp"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gclient.py",
       "runhooks"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "env": {
       "GYP_CHROMIUM_NO_ACTION": "1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gn.py",
       "--root=[SLAVE_BUILD]\\src",
       "gen",
       "//out/Debug_x64",
       "--args=is_debug=true target_cpu=\"x64\" use_goma=true goma_dir=\"[BUILD]\\goma\" build_with_chromium=false"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\compile.py",
       "--target",
       "Debug_x64",
       "--src-dir",
       "[SLAVE_BUILD]\\src",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma",
       "--goma-dir",
       "[BUILD]\\goma",
       "--",
       "all"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "compile"
   }
 ]