package_from_installed: Put env in json file behind "env" key.

Every time I write a json file, I end up wishing I put my toplevel items
into non-toplevel items down the line when I want to add more stuff to the
json file.  Address this now, while no toolchain with this json file has
been built yet.

Follow-up to https://codereview.chromium.org/1706423002/

BUG=495204

Review URL: https://codereview.chromium.org/1718083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298908 0039d316-1c4b-4281-b951-d872f2087c98

win_toolchain/package_from_installed.py

@@ -271,10 +271,12 @@ def GenerateSetEnvCmd(target_dir):
           var, BatDirs(dirs), ';%PATH%' if var == 'PATH' else ''))
   with open(set_env_prefix + '.x86.json', 'wb') as f:
     assert not set(env.keys()) & set(env_x86.keys()), 'dupe keys'
-    json.dump(collections.OrderedDict(env.items() + env_x86.items()), f)
+    json.dump({'env': collections.OrderedDict(env.items() + env_x86.items())},
+              f)
   with open(set_env_prefix + '.x64.json', 'wb') as f:
     assert not set(env.keys()) & set(env_x64.keys()), 'dupe keys'
-    json.dump(collections.OrderedDict(env.items() + env_x64.items()), f)
+    json.dump({'env': collections.OrderedDict(env.items() + env_x64.items())},
+              f)
 
 
 def AddEnvSetup(files):