Forward the whole Nix config to the post-build-hook

Fill `NIX_CONFIG` with the value of the current Nix configuration before
calling the post-build-hook.
That way the whole configuration (including the possible
`experimental-features`, a possibly `--store` option or whatever) will
be made available to the hook

src/nix/show-config.cc

@@ -22,10 +22,7 @@ struct CmdShowConfig : Command, MixJSON
             // FIXME: use appropriate JSON types (bool, ints, etc).
             logger->cout("%s", globalConfig.toJSON().dump());
         } else {
-            std::map<std::string, Config::SettingInfo> settings;
-            globalConfig.getSettings(settings);
-            for (auto & s : settings)
-                logger->cout("%s = %s", s.first, s.second.value);
+            logger->cout("%s", globalConfig.toKeyValue());
         }
     }
 };