Update cachix/install-nix-action action to v26

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
cachix/install-nix-action	action	major	v25 -> v26

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Release Notes

cachix/install-nix-action (cachix/install-nix-action)

### [`v26`](https://togithub.com/cachix/install-nix-action/releases/tag/v26) [Compare Source](https://togithub.com/cachix/install-nix-action/compare/v25...v26) Bump to Nix 2.20.5 to address CVE-2024-27297 / GHSA-2ffj-w4mj-pg37.

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