Bump cachix/install-nix-action from 14.1 to 15 (#2356)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 14.1 to 15.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v14.1...v15

)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: Javier Neira <atreyu.bbb@gmail.com>