Flake cleanup and make usable in a system configuration (#316)

Flake cleanup and make usable in a system configuration (#316)

* Do not rely on the flake registry to resolve nixpkgs

This can cause using the wrong nixpkgs if people have overwritten it
locally

* Misc cleanup

* Migrate off deprecated defaultPackage/defaultApp

* Don't install executable under $out/usr/bin

If the flake is used in a system configuration then now executable gets
linked and appears in PATH

* Update flake.nix

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Co-authored-by: b3nj5m1n <47924309+b3nj5m1n@users.noreply.github.com>