Update cachix/install-nix-action action to v21

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
cachix/install-nix-action	action	major	v20 -> v21

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Release Notes

cachix/install-nix-action

### [`v21`](https://togithub.com/cachix/install-nix-action/releases/tag/v21): install-nix-action-v21 [Compare Source](https://togithub.com/cachix/install-nix-action/compare/v20...v21) - pin Nix to 2.15.1 (recent releases broke too many things) - fix the action to work on custom containers

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