Automatically run renovate on a schedule

[joecorall]

I created a GitHub app to allow us to run renovate on this repo in GitHub Actions. "Creating an app" sounds a little deceiving. There's no code involved in creating a GitHub app. The creation is all driven through the GitHub UI. A private key associated to that app allows you to interact with the GitHub REST API as that app.

I tested the token works properly by running the script in this PR locally (./ci/fetch-app-token.sh app-id install-id privkey.pem) and set that token as RENOVATE_TOKEN. It created the PR #367

[joecorall]

huh I just noticed https://github.com/actions/create-github-app-token 🤔

UPDATE/EDIT:

On second thought, I tend to try to make any GitHub Action as reproducible as possible on my local machine to avoid getting into traps like needing to push to GitHub to test things in GitHub Actions. Though it's tempting to replace the bash script with the community supported GHA, I think I prefer the PR as is.