build(deps): bump cachix/install-nix-action from 25 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 25 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Changelog](https://github.com/cachix/install-nix-action/blob/master/RELEASE.md)
- [Commits](https://github.com/cachix/install-nix-action/compare/v25...v31)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-version: '31'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/nix-cache.yaml

@@ -16,7 +16,7 @@ jobs:
       - name: "CHeckout"
         uses: actions/checkout@v4
 
-      - uses: cachix/install-nix-action@v25
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable