Make env var eval order during "use" deterministic (#742)

* Make env var eval order during "use" deterministic

Fixes #726.

* Merge delta after getting config

To make sure env vars are all in the engine state and not in the stack.

crates/nu-command/src/system/run_external.rs

@@ -109,8 +109,6 @@ impl<'call> ExternalCommand<'call> {
 
         let ctrlc = engine_state.ctrlc.clone();
 
-        // TODO. We don't have a way to know the current directory
-        // This should be information from the EvaluationContex or EngineState
         let mut process = if let Some(d) = self.env_vars.get("PWD") {
             let mut process = self.create_command(d);
             process.current_dir(d);