allow for $in to affect environment (#6649)

* allow for `$in` to affect environment

* fix for vars, overlays, env_hidden

* fmt

* carry over variables properly

* add test

* modify name, remove redundant

* fmt

crates/nu-parser/src/parser.rs

@@ -5367,6 +5367,15 @@ fn wrap_expr_with_collect(working_set: &mut StateWorkingSet, expr: &Expression)
             custom_completion: None,
         }));
 
+        output.push(Argument::Named((
+            Spanned {
+                item: "keep-env".to_string(),
+                span: Span::new(0, 0),
+            },
+            None,
+            None,
+        )));
+
         // The containing, synthetic call to `collect`.
         // We don't want to have a real span as it will confuse flattening
         // The args are where we'll get the real info