Use GitHub App credentials for commits

These can be obtained using the following command:

export APP_ID=123456
export APP_SECRET="$(cat app.private-key.pem)"
export GH_REPO=owner/repo
export TOKEN=$(bash get_github_app_token.sh)
curl -X PUT \
    -H "Authorization: token ${TOKEN}" \
    -H "Accept: application/vnd.github.v3+json" \
    https://api.github.com/repos/$GH_REPO/contents/test-file \
    -d '{"message":"message","content":"Y29udGVudA==","branch":"test"}'

This will create a commit on branch `test`. The branch must exist.
The command will return a JSON that has wanted credentials.
This needs open-ssl, jq and curl, and the following script:

https://gist.github.com/987Nabil/594764a7444cb5eee0636607d06f43c5

.github/workflows/update.yml

@@ -37,8 +37,8 @@ jobs:
 
       - name: Prepare git
         run: |
-          git config user.name kittenoak
-          git config user.email kittenoak@users.noreply.github.com
+          git config user.name "anki-wallpaper[bot]"
+          git config user.email "105947902+anki-wallpaper[bot]@users.noreply.github.com"
 
       - name: Update pre-release test environment
         if: matrix.environment == 'pre-release'