Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/step/example.expected/defer_results.json

Index: scripts/slave/recipe_modules/step/example.expected/defer_results.json
diff --git a/scripts/slave/recipe_modules/step/example.expected/defer_results.json b/scripts/slave/recipe_modules/step/example.expected/defer_results.json
index 6701f548f6268907f33045816d2c200b97265be3..df47df583b619707518972617d421459f328d07b 100644
--- a/scripts/slave/recipe_modules/step/example.expected/defer_results.json
+++ b/scripts/slave/recipe_modules/step/example.expected/defer_results.json
@@ -4,6 +4,7 @@
       "echo",
       "Hello World"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "hello"
   },
   {
@@ -11,6 +12,7 @@
       "echo",
       "Why hello, there."
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "hello (2)"
   },
   {
@@ -19,6 +21,7 @@
       "-c",
       "echo Good bye, $friend."
     ],
+    "cwd": "[SLAVE_BUILD]",
     "env": {
       "friend": "Darth Vader"
     },
@@ -30,6 +33,7 @@
       "-c",
       "exit 3"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "anything is cool"
   },
   {
@@ -37,6 +41,7 @@
       "echo",
       "hello"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "hello (3)",
     "~followup_annotations": [
       "@@@STEP_EXCEPTION@@@"
@@ -47,6 +52,7 @@
       "echo",
       "goodbye"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "goodbye (2)"
   },
   {
@@ -54,6 +60,7 @@
       "echo",
       "testa"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "testa",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
@@ -65,6 +72,7 @@
       "echo",
       "testb"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "testb"
   },
   {