Update cachix/install-nix-action action to v29 (#863)

| datasource  | package                   | from | to  |
| ----------- | ------------------------- | ---- | --- |
| github-tags | cachix/install-nix-action | v27  | v29 |

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -166,7 +166,7 @@ jobs:
         if: github.ref_type == 'tag'
 
       - name: Install Nix
-        uses: cachix/install-nix-action@ba0dd844c9180cbf77aa72a116d6fbc515d0e87b # v27
+        uses: cachix/install-nix-action@9f70348d77d0422624097c4b7a75563948901306 # v29
         with:
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
         if: github.ref_type == 'tag'