Added nix flake for dev shell

[deephack1982]

Added a nix flake providing a development shell with the correct cc65 version. To use run the following command with nix installed.

nix develop

[inexorabletash]

Thank you for your interest, but I've learned the lesson that any package management stuff added to a project that the maintainer (i.e. me) doesn't use will quickly get broken and/or turn into a burden. This is pretty niche, so I won't be merging this.

I'd accept a PR that adds a flat file tracking the repo and git commit hash of dependencies (cc65, cadius), in conjunction with tooling updates to the github actions and some local tools that verify the versions, since that seems more broadly useful. Right now "tip of tree" is assumed by the actions; that works fine for me but it does technically mean builds are not reproducible.