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](cachix/install-nix-action@V28...v29)

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

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

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@V28
+    - uses: cachix/install-nix-action@v29
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: nix-build
@@ -23,7 +23,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@V28
+    - uses: cachix/install-nix-action@v29
       with:
         extra_nix_config: |
           access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}