The only user-facing changes is the process for version pinning, which will be
slightly less convenient without access to the `nix flake` feature. This is
justified by it rarely being necessary, for the benefit of relying on an
upcoming built-in nix feature rather than a non-standard third-party tool.
- deprecate `niv` for version pinning
- legacy nix now uses the flake via `flake-compat`
- remove `sources.{nix,json}` (versions now pinned in `flake.lock`)
- move contents of `nix/default.nix` to `nix/veloren.nix`
- make `nix-build nix/default.nix` produce the same output as `flake build`
- move contents of `nix/shell.nix` into `nix/devShell.nix`
- make `nix-shell nix/shell.nix` produce the same environment as `flake develop`
- Move `tag` parameter into `sourceInfo`
- Tidy up and autoformat with `nixpkgs-fmt`
- update README to reflect new usage
- revert input versions to match those previously specified in nix/sources.json
