CI: move nightly macOS builds from GA to pipelines (#32231)

Modifications:
- [x] Delete the `macOS builds nightly` workflow
- [x] Removed shield in README.md
- [x] Moved corresponding specs to `e4s-macos`

README.md

@@ -2,7 +2,6 @@
 
 [![Unit Tests](https://github.com/spack/spack/workflows/linux%20tests/badge.svg)](https://github.com/spack/spack/actions)
 [![Bootstrapping](https://github.com/spack/spack/actions/workflows/bootstrap.yml/badge.svg)](https://github.com/spack/spack/actions/workflows/bootstrap.yml)
-[![macOS Builds (nightly)](https://github.com/spack/spack/workflows/macOS%20builds%20nightly/badge.svg?branch=develop)](https://github.com/spack/spack/actions?query=workflow%3A%22macOS+builds+nightly%22)
 [![codecov](https://codecov.io/gh/spack/spack/branch/develop/graph/badge.svg)](https://codecov.io/gh/spack/spack)
 [![Containers](https://github.com/spack/spack/actions/workflows/build-containers.yml/badge.svg)](https://github.com/spack/spack/actions/workflows/build-containers.yml)
 [![Read the Docs](https://readthedocs.org/projects/spack/badge/?version=latest)](https://spack.readthedocs.io)