Use GITHUB_TOKEN instead of MOKAGIO_BOT_GITHUB_TOKEN

It's possible that my understanding of how GitHub Actions secrets work
and of which token to use if flawed. Maybe you need to use the same
exact syntax as the Danger docs suggest?

See danger/danger-js#856 (comment)

.github/workflows/danger-pr-labels.yml

@@ -24,4 +24,4 @@ jobs:
       - name: Check Labels with DangerJS
         run: yarn danger ci
         env:
-          GITHUB_TOKEN: ${{ secrets.MOKAGIO_BOT_GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}