CODEOWNERS: add bin/fetch-configlet (#296)

Each track repo has a `fetch-configlet` script that is a copy of the one
in the `configlet` repo.

We are therefore adding the path of the track copy to `CODEOWNERS` to
prevent the track from changing it independently. A contributor should
propose changes against the upstream copy so that any update can benefit
every track.

See:
- https://github.com/exercism/configlet/blob/main/scripts/fetch-configlet
- exercism/configlet#286

.github/CODEOWNERS

@@ -1,5 +1,8 @@
-# Maintainers 
+# Maintainers
 config/maintainers.json @exercism/maintainers-admin
 
 # Code owners
 .github/CODEOWNERS      @exercism/maintainers-admin
+
+# Changes to `fetch-configlet` should be made in the `exercism/configlet` repo
+bin/fetch-configlet     @exercism/maintainers-admin