Bump cachix/install-nix-action from 25 to 26 (#9557)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 25 to 26.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v25...v26)

---
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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/ci.yml

@@ -145,7 +145,7 @@ jobs:
       fail-fast: true
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@v25
+    - uses: cachix/install-nix-action@v26
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: Cache cabal global package db