Bump cachix/install-nix-action from V27 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from V27 to 31. This release includes the previously tagged commit.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@V27...v31)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/test.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/[email protected]
-      - uses: cachix/install-nix-action@V27
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - run: nix flake check

.github/workflows/update.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/[email protected]
-      - uses: cachix/install-nix-action@V27
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - run: ./update