[step_runner] run _merge_envs in simulation too.

recipe_modules/context/examples/full.py

Index: recipe_modules/context/examples/full.py
diff --git a/recipe_modules/context/examples/full.py b/recipe_modules/context/examples/full.py
index 4a9c13ff8b3d29e8cca3211f3db6e811879cbd08..9a74c96235d2ea528ed6b336251a10ff99820162 100644
--- a/recipe_modules/context/examples/full.py
+++ b/recipe_modules/context/examples/full.py
@@ -30,6 +30,18 @@ def RunSteps(api):
   with api.context(env={"HELLO": "WORLD", "HOME": None}):
     api.step('env step', ['bash', '-c', 'echo $HELLO; echo $HOME'])
 
+  # %-formats are errors (for now). Double-% escape them.
+  bad_examples = ["%format", "%s"]
+  for example in bad_examples:
+    try:
+      with api.context(env={"BAD": example}):
+        assert False  # pragma: no cover
+    except ValueError:
+      pass
+
+  # this is fine though:
+  api.context(env={"FINE": "%%format"})
+
   # can increment nest level... note that this is a low level api, prefer
   # api.step.nest instead:
   # YES: