Fix misuse of update() in run.py (#1245)

jsmodules/webpack-config/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@attic/webpack-config",
-  "version": "2.0.0",
+  "version": "2.1.0",
   "description": "Shared webpack config for noms",
   "main": "index.js",
   "dependencies": {

jsmodules/webpack-config/run.py

@@ -12,7 +12,8 @@ def main():
     env = {
         'NODE_ENV': args.mode,
         'BABEL_ENV': args.mode,
-    }.update(os.environ)
+    }
+    env.update(os.environ)
     subprocess.check_call(
             ['node_modules/.bin/webpack',
                 '--config', 'node_modules/@attic/webpack-config/index.js', args.src, args.out],