Avoid uses of Record internals again (#12312)

# Description
Again avoid uses of the `Record` internals, so we are free to change the
data layout

- **Don't use internals of `Record` in `into sqlite`**
- **Don't use internals of `Record` in `to xml`**

Remaining: `rename`

# User-Facing Changes
None

crates/nu-command/src/formats/to/xml.rs

@@ -269,8 +269,7 @@ impl Job {
     fn find_invalid_column(record: &Record) -> Option<&String> {
         const VALID_COLS: [&str; 3] = [COLUMN_TAG_NAME, COLUMN_ATTRS_NAME, COLUMN_CONTENT_NAME];
         record
-            .cols
-            .iter()
+            .columns()
             .find(|col| !VALID_COLS.contains(&col.as_str()))
     }