Add flakehub publish workflow

[elpdt852]

Using: https://flakehub.com/new

This will make the pdtpartners/nix-snapshotter flake listed on FlakeHub.

Our flake will become available under the following references in addition to github:pdtpartners/nix-snapshotter:

• Latest stable: https://flakehub.com/f/pdtpartners/nix-snapshotter/*.tar.gz
• Specific version: https://flakehub.com/f/pdtpartners/nix-snapshotter/0.1.0.tar.gz