Recipes: make cwd default to slave_build

scripts/slave/recipes/libvpx/unittests.expected/basic_linux_64.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys, shutil\nroot = sys.argv[1]\nnuke_dirs = sys.argv[2:]\nfor fname in os.listdir(root):\n  path = os.path.join(root, fname)\n  if os.path.isfile(path):\n    os.unlink(path)\n  elif fname in nuke_dirs:\n    shutil.rmtree(path)\n",
       "[SLAVE_BUILD]",
       "libs",
       "obj",
       "vp8",
       "vp9",
       "vpx",
       "vpx_mem",
       "vpx_ports",
       "vpx_scale",
       "third_party"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "clean_build",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import os, sys, shutil@@@",
       "@@@STEP_LOG_LINE@python.inline@root = sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@nuke_dirs = sys.argv[2:]@@@",
       "@@@STEP_LOG_LINE@python.inline@for fname in os.listdir(root):@@@",
       "@@@STEP_LOG_LINE@python.inline@  path = os.path.join(root, fname)@@@",
       "@@@STEP_LOG_LINE@python.inline@  if os.path.isfile(path):@@@",
       "@@@STEP_LOG_LINE@python.inline@    os.unlink(path)@@@",
       "@@@STEP_LOG_LINE@python.inline@  elif fname in nuke_dirs:@@@",
       "@@@STEP_LOG_LINE@python.inline@    shutil.rmtree(path)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/git_setup.py",
       "--path",
       "[SLAVE_BUILD]/libvpx",
       "--url",
       "https://chromium.googlesource.com/webm/libvpx"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "git setup"
   },
   {
     "cmd": [
       "git",
       "retry",
       "fetch",
       "origin",
       "master",
       "--recurse-submodules"
@@ skipping 39 matching lines @@
       "--init",
       "--recursive"
     ],
     "cwd": "[SLAVE_BUILD]/libvpx",
     "name": "submodule update"
   },
   {
     "cmd": [
       "./libvpx/configure"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "configure"
   },
   {
     "cmd": [
       "make",
       "test",
       "-j8"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "run tests"
   }
 ]