Improve nightly builds

[teodanciu]

Description

A few improvements to nightly builds:

• Rename the Github action run to make it clear it's a nightly build - hopefully this will make it easier to distinguish it in the emails if it fails
• Add new substituters (so that we can use caching in github actions running nix )
• Run nix-build and nix-shell as part of nightly builds, so we are aware at latest the next day if they've been broken

Checklist

• Commit sequence broadly makes sense
• Commits have useful messages
• New tests are added if needed and existing tests are updated
• Any changes are noted in the changelog
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[JaredCorduan]

thank you!!