Configure Nix in Dependabot

[BD103]

Dependabot now supports Nix. We should configure it to automatically upgrade dependencies. The docs for Nix aren't live yet, but the PR for them is open.

[nmpiazza]

So my PR #836 to set this up works. #853 was properly kicked off, but ran into an error preventing merge.

It looks like nixpkgs 26.11 will drop support for x86_64-darwin.
https://github.com/TheBevyFlock/bevy_cli/actions/runs/29353026778/job/87153717399?pr=853#step:4:48

Is this system one that this project is looking to support long term?