Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/gsutil/example.expected/basic.json

Index: scripts/slave/recipe_modules/gsutil/example.expected/basic.json
diff --git a/scripts/slave/recipe_modules/gsutil/example.expected/basic.json b/scripts/slave/recipe_modules/gsutil/example.expected/basic.json
index b44b5a9dbb5d27529d2a2f536e565d5e86909e27..46b22300785a28204d29cc10e82bf67f30bce0c0 100644
--- a/scripts/slave/recipe_modules/gsutil/example.expected/basic.json
+++ b/scripts/slave/recipe_modules/gsutil/example.expected/basic.json
@@ -21,6 +21,7 @@
       "[SLAVE_BUILD]/boom",
       "gs://chromium-recipe-test/some/random/path/to/boom"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil upload",
     "~followup_annotations": [
       "@@@STEP_LINK@gsutil.upload@https://storage.googleapis.com/chromium-recipe-test/some/random/path/to/boom@@@"
@@ -40,6 +41,7 @@
       "gs://chromium-recipe-test/some/random/path/**",
       "gs://chromium-recipe-test/staging"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil cp"
   },
   {
@@ -56,6 +58,7 @@
       "gs://chromium-recipe-test/some/random/path/**",
       "gs://chromium-recipe-test/staging"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil cp (2)"
   },
   {
@@ -72,6 +75,7 @@
       "gs://chromium-recipe-test/some/random/path/to/boom",
       "[SLAVE_BUILD]/boom"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil gsutil download url"
   },
   {
@@ -88,6 +92,7 @@
       "gs://chromium-recipe-test/staging/to/boom",
       "[SLAVE_BUILD]/erang"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil download"
   },
   {
@@ -104,6 +109,7 @@
       "path/to/key",
       "gs://chromium-recipe-test/some/random/path/to/boom"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil signed url"
   },
   {
@@ -119,6 +125,7 @@
       "rm",
       "gs://chromium-recipe-test/staging/to/boom"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil remove"
   },
   {