Bump cachix/install-nix-action from V28 to 29

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from V28 to 29. This release includes the previously tagged commit.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/V28...v29)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/cachix.yml

@@ -35,7 +35,7 @@ jobs:
         with:
           ref: ${{ inputs.ref }}
       - name: Install Nix
-        uses: cachix/install-nix-action@V28
+        uses: cachix/install-nix-action@v29
         with:
           github_access_token: '${{ secrets.GITHUB_TOKEN }}'
       - name: Enable Cachix