snapcraft: depends_on :macos (#19460)
snapcraft: depends_on :macos (#19460)

- A new attempt at building it didn't work. Something about python-apt,
  Sphinx and `--single-version-externally-managed`.
- So, in the nicest possible way, I'm done. :-)
- If someone wants to fix it, be my guest! There's prior attempts
  thoroughly documented in
  https://github.com/Homebrew/linuxbrew-core/pull/14123.
rwhogg/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core