perf(ci): use self-hosted runner for bors (#8024)
Run CI builds for the `staging` branch used by bors on a self-hosted github actions runner. This runner has been graciously provided by Johan Commelin's DFG grant and is hosted at the Albert-Ludwigs-Universität Freiburg.
We need to use two github actions workflow files to use a separate runner for the `staging` branch: `build.yml` and `bors.yml`. These are almost identical, except for the `runs-on:` property indicating which runner should be used. The shell script `mk_build_yml.sh` is therefore used to generate both files from the common template `build.yml.in`.
