Bump cachix/install-nix-action from 27 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 27 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v27...v31

)

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

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

.github/workflows/nix.yml

@@ -18,7 +18,7 @@ jobs:
         os: [ubuntu, macos]
     steps:
       - uses: actions/checkout@v4
-      - uses: cachix/install-nix-action@v27
+      - uses: cachix/install-nix-action@v31
       - uses: cachix/cachix-action@v16
         with:
           name: gepetto