Bump cachix/install-nix-action from 27 to 29

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 27 to 29.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@ba0dd84...9f70348)

---
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]>

.github/workflows/nix-test.yaml

@@ -10,7 +10,7 @@ jobs:
       with:
         fetch-depth: 0
     - name: Install Nix
-      uses: cachix/install-nix-action@ba0dd844c9180cbf77aa72a116d6fbc515d0e87b # v27
+      uses: cachix/install-nix-action@9f70348d77d0422624097c4b7a75563948901306 # v29
       with:
         extra_nix_config: |
           access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}