Create and synchronize a PR in a dependent repository

# Hacking

```sh
git clone https://code.forgejo.org/actions/setup-forgejo
export PATH=$(pwd)/setup-forgejo:$PATH
git clone https://code.forgejo.org/actions/cascading-pr
cd cascading-pr
forgejo-curl.sh logout
forgejo-runner.sh teardown
forgejo.sh teardown
forgejo.sh setup root admin1234 codeberg.org/forgejo/forgejo 1.21
forgejo-runner.sh setup
firefox http://$(cat forgejo-ip):3000
tests/run.sh --debug
```