Recipes: make cwd default to slave_build

scripts/slave/recipes/skia/infra.expected/Infra-PerCommit_initialcheckout.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",
       "[SLAVE_BUILD]/go/src",
       "511"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "makedirs makedirs go/src",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys, os@@@",
       "@@@STEP_LOG_LINE@python.inline@path = sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@mode = int(sys.argv[2])@@@",
       "@@@STEP_LOG_LINE@python.inline@if not os.path.isdir(path):@@@",
       "@@@STEP_LOG_LINE@python.inline@  if os.path.exists(path):@@@",
       "@@@STEP_LOG_LINE@python.inline@    print \"%s exists but is not a dir\" % path@@@",
       "@@@STEP_LOG_LINE@python.inline@    sys.exit(1)@@@",
       "@@@STEP_LOG_LINE@python.inline@  os.makedirs(path, mode)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "cmd": [
       "git",
       "clone",
       "https://skia.googlesource.com/buildbot",
       "[SLAVE_BUILD]/go/src/go.skia.org/infra"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "git clone"
   },
   {
     "cmd": [
       "git",
       "fetch",
       "origin"
     ],
     "cwd": "[SLAVE_BUILD]/go/src/go.skia.org/infra",
     "name": "git fetch"
@@ skipping 93 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]/go/src/go.skia.org/infra",
     "env": {
       "GIT_USER_AGENT": "git/1.9.1",
       "GOPATH": "[SLAVE_BUILD]/go",
       "PATH": "[SLAVE_BUILD]/go/bin:%(PATH)s"
     },
     "name": "run_unittests"
   }
 ]