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] <[email protected]>

.github/workflows/build-with-nix.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ${{matrix.os}}
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@V28
+    - uses: cachix/install-nix-action@v29
     - uses: cachix/cachix-action@v15
       with:
         name: tket
@@ -32,7 +32,7 @@ jobs:
   #  runs-on: nixos-gpu
   #  steps:
   #  - uses: actions/checkout@v4
-  #  - uses: cachix/install-nix-action@V28
+  #  - uses: cachix/install-nix-action@v29
   #  - uses: cachix/cachix-action@v15
   #    with:
   #      name: tket