chore(ci): bump cachix/install-nix-action from 25 to 26 (#490)

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](cachix/install-nix-action@6004951...8887e59)

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

Author: dependabot[bot]

.github/workflows/nix-shell.yml

@@ -31,7 +31,7 @@ jobs:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
         with:
           submodules: true
-      - uses: cachix/install-nix-action@6004951b182f8860210c8d6f0d808ec5b1a33d28 # v25
+      - uses: cachix/install-nix-action@8887e596b4ee1134dae06b98d573bd674693f47c # v26
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - uses: rrbutani/use-nix-shell-action@59a52b2b9bbfe3cc0e7deb8f9059abe37a439edf # v1.1.0