chore(ci): bump cachix/install-nix-action from 30 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 30 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@08dcb3a...91a0719)

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

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

Author: dependabot[bot]

.github/workflows/nix-shell.yml

@@ -31,7 +31,7 @@ jobs:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           submodules: true
-      - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+      - uses: cachix/install-nix-action@91a071959513ca103b54280ac0bef5b825791d4d # v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - uses: rrbutani/use-nix-shell-action@59a52b2b9bbfe3cc0e7deb8f9059abe37a439edf # v1.1.0