Do not pass non-string env vars to externals (#4748)

* Do not pass non-string env vars to externals

Also misc cleanup

* Add note to default config

* Add a test

* Ensure PATH/Path conversion list <-> string

crates/nu-command/src/viewers/griddle.rs

@@ -66,7 +66,7 @@ prints out the list properly."#
         let separator_param: Option<String> = call.get_flag(engine_state, stack, "separator")?;
         let config = stack.get_config().unwrap_or_default();
         let env_str = match stack.get_env_var(engine_state, "LS_COLORS") {
-            Some(v) => Some(env_to_string("LS_COLORS", v, engine_state, stack, &config)?),
+            Some(v) => Some(env_to_string("LS_COLORS", &v, engine_state, stack)?),
             None => None,
         };
         let use_grid_icons = config.use_grid_icons;