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

recipe_modules/step/example.expected/infra_failure.json

Index: recipe_modules/step/example.expected/infra_failure.json
diff --git a/recipe_modules/step/example.expected/infra_failure.json b/recipe_modules/step/example.expected/infra_failure.json
index aaa429644be4d1effb8ee124560c600ead828c69..cf308873dd5e64b977cfc467c356ac0f9f0cf3ea 100644
--- a/recipe_modules/step/example.expected/infra_failure.json
+++ b/recipe_modules/step/example.expected/infra_failure.json
@@ -4,7 +4,6 @@
       "echo",
       "Hello World"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "hello"
   },
   {
@@ -12,7 +11,6 @@
       "echo",
       "Why hello, there."
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "hello (2)"
   },
   {
@@ -21,7 +19,6 @@
       "-c",
       "echo Good bye, $friend."
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "friend": "Darth Vader"
     },
@@ -32,7 +29,6 @@
       "recipes.py",
       "--help"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "RECIPE_PACKAGE_REPO[recipe_engine]:%(PATH)s"
     },
@@ -44,7 +40,6 @@
       "-c",
       "exit 3"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "anything is cool"
   },
   {
@@ -52,7 +47,6 @@
       "echo",
       "hello"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "hello (3)",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@the reason@The reason@@@",
@@ -63,7 +57,6 @@
   },
   {
     "cmd": [],
-    "cwd": "[SLAVE_BUILD]",
     "name": "Just print stuff",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@more@More stuff@@@",
@@ -75,7 +68,6 @@
       "echo",
       "goodbye"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "goodbye (2)"
   },
   {
@@ -83,7 +75,6 @@
       "echo",
       "warning"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "warning"
   },
   {
@@ -91,7 +82,6 @@
       "echo",
       "testa"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "testa"
   },
   {
@@ -99,7 +89,6 @@
       "echo",
       "testb"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "testb"
   },
   {
@@ -109,7 +98,6 @@
       "up",
       "build"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "cleanup",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
@@ -125,7 +113,6 @@
       "main",
       "application"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "application"
   },
   {