Add Nix language and snippet for fetch pkgs functions

[sea7ones]

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• 🐛 Bug Fix: Fixes a bug or issue.
• ✨ New Feature: Adds a new feature or functionality.
• 📚 Snippet Related: Contributes a code snippet.
• ⚙️ Other (please describe):

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• I’ve thoroughly tested my changes.
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[itsbrunodev]

Thanks for submitting this snippet!