Bump cachix/install-nix-action from 25 to 26

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>

.github/workflows/ci.yml

@@ -67,7 +67,7 @@ jobs:
 
       - name: Set up nix
         if: matrix.type == 'dapp'
-        uses: cachix/install-nix-action@v25
+        uses: cachix/install-nix-action@v26
 
       - name: Set up cachix
         if: matrix.type == 'dapp'