Fix configlet-sync: add missing GitHub token

The configlet-ci action switched to using the gh cli command and needs
to have `GH_TOKEN` set to work now.

This sets the token in the same way as upstream does it at
https://github.com/exercism/github-actions/blob/a3f83169a9218d28cae3c04d70522f0519d92d46/.github/workflows/configlet.yml#L29-L33

Previous error was:

> The GH_TOKEN environment variable is not set

See exercism/github-actions#107 for more context

.github/workflows/configlet-sync.yml

@@ -15,7 +15,12 @@ jobs:
 
     - name: Fetch configlet
       uses: exercism/github-actions/configlet-ci@main
-
+      # GITHUB_TOKEN is not set when we run the fetch script, because we're in
+      # a composite action. Set GH_TOKEN so that `gh release download` can
+      # make authenticated requests (it fails otherwise).
+      env:
+        GH_TOKEN: ${{ github.token }}
+
     - name: Configlet Sync
       id: sync
       shell: bash {0}