recipe engine: extract infra-specific paths out of the engine

recipes/example/nested.expected/basic.json

Index: recipes/example/nested.expected/basic.json
diff --git a/recipes/example/nested.expected/basic.json b/recipes/example/nested.expected/basic.json
index e25856e592f5357a459b6315a28f70afe3549b3c..c816ed95dd5195a6bf2bc444f9334ead2d50146a 100644
--- a/recipes/example/nested.expected/basic.json
+++ b/recipes/example/nested.expected/basic.json
@@ -1,12 +1,10 @@
 [
   {
     "cmd": [],
-    "cwd": "[SLAVE_BUILD]",
     "name": "complicated thing"
   },
   {
     "cmd": [],
-    "cwd": "[SLAVE_BUILD]",
     "name": "complicated thing.first part",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
@@ -17,7 +15,6 @@
       "sleep",
       "10"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "complicated thing.first part.wait a bit",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@2@@@"
@@ -28,7 +25,6 @@
       "echo",
       "herpy"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "complicated thing.attempt number.one",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
@@ -39,7 +35,6 @@
       "echo",
       "derpy"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "complicated thing.attempt number.two",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
@@ -50,7 +45,6 @@
       "sleep",
       "10"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "simple thing"
   },
   {