Recipes: make cwd default to slave_build

scripts/slave/recipes/example/defer_multiple.expected/nested_aggregate_fail.json

Index: scripts/slave/recipes/example/defer_multiple.expected/nested_aggregate_fail.json
diff --git a/scripts/slave/recipes/example/defer_multiple.expected/nested_aggregate_fail.json b/scripts/slave/recipes/example/defer_multiple.expected/nested_aggregate_fail.json
index 758e4605a41cdf9ef121c6a62e138eee10567fed..974740d74dd64178aed43c3ed45d441a08f26ddd 100644
--- a/scripts/slave/recipes/example/defer_multiple.expected/nested_aggregate_fail.json
+++ b/scripts/slave/recipes/example/defer_multiple.expected/nested_aggregate_fail.json
@@ -3,12 +3,14 @@
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "prelude"
   },
   {
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "aggregated start",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
@@ -19,36 +21,42 @@
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "aggregated finish"
   },
   {
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "normal start"
   },
   {
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "normal finish"
   },
   {
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "composite start"
   },
   {
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "composite finish"
   },
   {
     "cmd": [
       "true"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "clean up"
   },
   {