Do not set user defined env variables twice for (c)make

These variables are already exported in the build script prelude and
thus don't have to be set on individual make commands.

foreign_cc/private/make_env_vars.bzl

@@ -123,10 +123,8 @@ def _get_make_variables(workspace_name, tools, flags, user_env_vars):
 
     vars.update(tools_dict)
 
-    # Put all other environment variables, passed by the user
-    for user_var in user_env_vars:
-        if not vars.get(user_var):
-            vars[user_var] = [user_env_vars[user_var]]
+    # Do not put in the other user-defined env variables at this point as they
+    # have already been exported globally by the prelude.
 
     return vars