webrtc: use correct cwd for running tests

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]\\win_baremetal",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 203 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::webrtc]\\resources\\ensure_webcam_is_running.py"
     ],
+    "cwd": "[BUILDER_CACHE]\\win_baremetal",
     "name": "webcam_check"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"win_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"win_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=audio_device_tests",
       "--step-name=audio_device_tests",
       "--builder-name=win_baremetal",
       "--slave-name=slavename",
       "--build-number=1337",
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "audio_device_tests.exe"
     ],
+    "cwd": "[BUILDER_CACHE]\\win_baremetal",
     "name": "audio_device_tests"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"win_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"win_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=voe_auto_test",
       "--step-name=voe_auto_test",
       "--builder-name=win_baremetal",
       "--slave-name=slavename",
       "--build-number=1337",
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "voe_auto_test.exe",
       "--automated"
     ],
+    "cwd": "[BUILDER_CACHE]\\win_baremetal",
     "name": "voe_auto_test"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"win_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"win_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=video_capture_tests",
       "--step-name=video_capture_tests",
       "--builder-name=win_baremetal",
       "--slave-name=slavename",
       "--build-number=1337",
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "video_capture_tests.exe"
     ],
+    "cwd": "[BUILDER_CACHE]\\win_baremetal",
     "name": "video_capture_tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]