refactor(turborepo-updater): chain environment variable iterators in should_skip_notification (#10622)
## Description
Optimizes the `should_skip_notification()` function by combining two
separate environment variable checks into a single chained iterator
operation.
### Changes
- **Before**: Two separate `.any()` calls on `NOTIFIER_DISABLE_VARS` and
`ENVIRONMENTAL_DISABLE_VARS`
- **After**: Single chained iterator using `.chain()` method
### Benefits
- **Improved readability**: Cleaner, more idiomatic Rust code
- **Reduced redundancy**: Eliminates duplicate iteration logic
- **Better maintainability**: Follows Rust iterator best practices
- **Same functionality**: Zero behavioral changes