Support `shell.nix` or `default.nix` in a different location

[cgrindel]

### Tasks
- [ ] #5
- [ ] tweag/rules_nixpkgs/pull/468
- [x] Create a release for `run-nix-shell`. https://github.com/tweag/run-nix-shell/releases/tag/v0.2.0
- [ ] tweag/rules_haskell/pull/2083