Add NixOS support

[abdallah-azzouni]

Role

As a NixOS user, it will be really helpful to install pollymc from package manager.

Suggestion

Add pollymc in the NixOS package manager system.

Benefit

To make it easier for NixOS users to install and enjoy Pollymc.

This suggestion is unique

• [X] I have searched the issue tracker and did not find an issue describing my suggestion, especially not one that has been rejected.

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