chore(ci.deps): bump cachix/install-nix-action from 26 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 26 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v26...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] <[email protected]>

Author: dependabot[bot]

.github/workflows/update_flake.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0 # Required to count the commits
-      - uses: cachix/install-nix-action@v26
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - name: Run flake-update