This allows construction of `rust-bin` attrset on an existing `pkgs`,
instead of staging another overlayed nixpkgs. This can reduce redundant
evaluation of nixpkgs for flake-heavy users who also don't like or
cannot (due to flake dependency hell) manually instantiate nixpkgs.
This reverts commit cf102ae729, reversing
changes made to ffe47b9007.
Nix doesn't allow a dependent flake to have relative inputs.
```
error: cannot fetch input 'path:./systems.nix?lastModified=1&narHash=sh
a256-cyfZqnwhPNzdfQnmUUUVK5fon88TSm/0CwgE7Lf%2BLDU%3D' because it uses a relat
ive path
```
Closes#125