path: Add tracking of volatile paths.

recipe_modules/path/example.expected/mac_kitchen.json

 [
   {
     "cmd": [
       "/bin/echo",
       "[TMP_BASE]/foo"
     ],
     "name": "step1"
   },
   {
     "cmd": [
@@ skipping 26 matching lines @@
   {
     "cmd": [
       "touch",
       "[HOME]/file"
     ],
     "name": "touch my home"
   },
   {
     "cmd": [
       "echo",
+      "False"
+    ],
+    "name": "volatile tmp_dir"
+  },
+  {
+    "cmd": [
+      "echo",
       "[START_DIR]/some/thing"
     ],
     "name": "converted path [START_DIR]/some/thing"
   },
   {
     "cmd": [
       "echo",
       "[START_DIR]"
     ],
     "name": "converted path [START_DIR]"
@@ skipping 25 matching lines @@
       "RECIPE[recipe_engine::path:example].resources"
     ],
     "name": "converted path RECIPE[recipe_engine::path:example].resources"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]