actions: 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/ci.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: cachix/install-nix-action@V28
+      - uses: cachix/install-nix-action@v29
 
       - uses: cachix/cachix-action@v15
         with:

.github/workflows/update-flake.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: cachix/install-nix-action@V28
+      - uses: cachix/install-nix-action@v29
         with:
           github_access_token: ${{ github.token }}