Uninstall deprecated magic-nix-cache-action

https://determinate.systems/posts/magic-nix-cache-free-tier-eol/

Author: kachick

.github/workflows/ci-nix.yml

@@ -33,7 +33,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@v16
-      - uses: DeterminateSystems/magic-nix-cache-action@v9
       - run: nix flake check
       - run: nix develop --command echo 'This step should be done before any other "nix develop" steps because of measuring Nix build time'
       - run: nix develop --command bundle install