build: added shell.nix

[caarlos0]

adds a shell.nix with most of what's needed to work on nfpm

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a8707cd) 75.18% compared to head (9223fb7) 75.28%. Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #743 +/- ## ========================================== + Coverage 75.18% 75.28% +0.10% ========================================== Files 10 10 Lines 2422 2432 +10 ========================================== + Hits 1821 1831 +10 Misses 425 425 Partials 176 176 ```

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