Remove nix store caching - no suitable approach found with the permis…

…sions abilities and the target matrix that we'll need. Github's own actions/cache runs into permissions issues and fails to rebuild the nix store. Determinate Systems' magic nix cache relies on a download that doesn't exist for ARM macos. Long term we should investigate using a public cache (e.g. cachix or host our own) that not only speeds up CI, but also speeds up the process of installing tket for users

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

@@ -23,6 +23,5 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@v23
-    - uses: DeterminateSystems/magic-nix-cache-action@v2
     - name: Build and test tket
       run: nix flake check -L