Avoid cancelling scheduled CI when a new merge happens on master

[neilmayhew]

Description

If a PR is merged to master before a nightly CI run has finished, the nightly will be cancelled because it's in the same concurrency group and we have cancel-in-progress set.

This PR puts scheduled workflows into a separate set of concurrency groups from those triggered by other events, so that they don't conflict with each other.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• 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

[lehins]

Very nice! Thank you!