Recipes: make cwd default to slave_build

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

Index: scripts/slave/recipe_modules/filter/example.expected/basic.json
diff --git a/scripts/slave/recipe_modules/filter/example.expected/basic.json b/scripts/slave/recipe_modules/filter/example.expected/basic.json
index a65a2526634c7a709bd1ac480c33541ff79aa539..b273703c3587795c74c3136d9b00a5838fca7084 100644
--- a/scripts/slave/recipe_modules/filter/example.expected/basic.json
+++ b/scripts/slave/recipe_modules/filter/example.expected/basic.json
@@ -7,6 +7,7 @@
       "[SLAVE_BUILD]/testing/buildbot/trybot_analyze_config.json",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "read filter exclusion spec",
     "~followup_annotations": [
       "@@@STEP_TEXT@path: 'testing/buildbot/trybot_analyze_config.json'@@@",
@@ -30,6 +31,7 @@
       "{\"files\": [\"yy\"], \"targets\": []}",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "env": {
       "GYP_DEFINES": "component=static_library gomadir='[BUILD]/goma' target_arch=x64 use_goma=1"
     },
@@ -49,6 +51,7 @@
       "echo",
       "Why hello, there."
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "hello"
   }
 ]