Introduce nightly build for incremental but full recipe coverage

This is to discuss a feature to implement in order to have a full recipe coverage build time.
The idea is to compile every nights a subset of our recipes.
```
name: Nightly build
on:
schedule:
- cron: '0 2 * * *' # run at 2 AM UTC

jobs:
nightly:
name: Nightly build
...
```
This covers builds time checks first and may cover runtime checks in the future.

Start with the proposed GitHub Actions workflow and its nightly schedule, then determine how the recipes should be divided into nightly subsets. Done means a nightly build compiles every recipe over time and provides build-time coverage checks, with runtime checks left for future work.

Written by the indexing model from the issue text.