Chez Scheme libraries are broken. For example `guix environment --ad-hoc
chez-scheme chez-fmt`. chez-fmt will not install (for me) because it is
dependent on chez-srfi and chez-scheme seem unable to locate it.

I think it is because CHEZSCHEMELIBDIRS is not being set. But I'm not
sure why that is happening. The chez-scheme package includes the following


```
(native-search-paths
      (list (search-path-specification
            (variable "CHEZSCHEMELIBDIRS")
            (files (list (string-append "lib/csv" version "-site"))))))
```
Which I think should set it.

-Martin