Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/commit_position/example.expected/valid.json

 [
   {
     "cmd": [
       "/bin/echo",
       "refs/heads/master"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "test branch parse"
   },
   {
     "cmd": [
       "/bin/echo",
       "12345"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "test revision parse"
   },
   {
     "cmd": [
       "/bin/echo",
       "refs/heads/master@{#12345}"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "test construction"
   }
 ]