Avoid BorrowMutError with RUSTC_LOG=debug
```console
$ touch empty.rs
$ env RUSTC_LOG=debug rustc +stage1 --crate-type=lib empty.rs
```
Fails with a `BorrowMutError` because source map files are already
borrowed while `features_query` attempts to format a log message
containing a span.
Release the borrow before the query to avoid the issue.