We have a limited number of macOS workers (for the Homebrew
organisation) and queuing these seems to be slowing things down overall
compared to when we had one big job.

Linux throughput is still significantly quicker so it makes sense to
split up those jobs for providing quick feedback on breakages that are
cross-platform (which ends up being most but definitely not all of
them).

All existing workflows using this seem to be broken.

Separate out the slow parts of tests into jobs that can be run in
parallel.

Also:
- general refactoring from YAML linter.
- don't build on `macOS-latest` now that Big Sur is the latest stable
  release.
- Use `runner.os` when it improves readability over `matrix.os`.

Needed after https://github.com/Homebrew/homebrew-test-bot/pull/526.

Bumps [reitermarkus/rerun-workflow](https://github.com/reitermarkus/rerun-workflow) from cf91bee6964dfde64eccbf5600c3ea206af11359 to 1.1.5. This release includes the previously tagged commit.
- [Release notes](https://github.com/reitermarkus/rerun-workflow/releases)
- [Commits](https://github.com/reitermarkus/rerun-workflow/compare/cf91bee6964dfde64eccbf5600c3ea206af11359...e2647e8885422412d5acbd61f9add5b1e77ad3ab

)

Signed-off-by: dependabot[bot] <support@github.com>

Remove `HOMEBREW_BIG_SUR_TESTING` variable as it is no longer needed.

Migrate GitHub Actions to Big Sur.

https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

> Action and workflow authors who are setting environment variables via stdout should update any usage of the set-env and add-path workflow commands to use the new environment files.