Bump cachix/install-nix-action from 26 to 29

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 26 to 29.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v26...v29)

---
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>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -145,7 +145,7 @@ jobs:
             matrix: ${{ steps.set-matrix.outputs.matrix }}
         steps:
             -   uses: actions/checkout@v4
-            -   uses: cachix/install-nix-action@v26
+            -   uses: cachix/install-nix-action@v29
             -   id: set-matrix
                 name: Generate Nix Matrix
                 run: |
@@ -173,7 +173,7 @@ jobs:
                         nix-store.nar-${{ runner.os }}-${{ hashFiles('flake.lock') }}-
                         nix-store.nar-${{ runner.os }}-
 
-            -   uses: cachix/install-nix-action@v26
+            -   uses: cachix/install-nix-action@v29
                 with:
                     nix_path: nixpkgs=channel:nixos-24.05
                     extra_nix_config: "system-features = nixos-test benchmark big-parallel kvm"