configure a user even if using signed commits

Even when using signed commits, there are local commits created which
need some kind of author information. When using a docker image, there
may not be anything configured. For consistency with the API, configure
author information.

Author: haarg

.github/workflows/update-snapshot.yml

@@ -15,6 +15,9 @@ jobs:
         with:
           app-id: ${{ secrets.APP_ID }}
           private-key: ${{ secrets.APP_PRIVATE_KEY }}
+      - uses: haarg/setup-git-user@v1
+        with:
+          app: ${{ steps.app-token.output.app-slug }}
       - uses: actions/checkout@v4
         with:
           token: ${{ steps.app-token.outputs.token }}