fix(ci): Remove Cachix binary cache step

The Cachix binary cache 'openpfc' doesn't exist or lacks proper
authentication. Removed the cachix-action step to fix CI failures.

Builds will still work but won't benefit from binary caching.

Author: ahojukka5

.github/workflows/ci.yml

@@ -67,14 +67,6 @@ jobs:
         with:
           nix_path: nixpkgs=channel:nixos-unstable
 
-      - name: Setup Cachix (optional, speeds up builds)
-        uses: cachix/cachix-action@v14
-        with:
-          name: openpfc
-          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-          skipPush: ${{ github.event_name == 'pull_request' }}
-        continue-on-error: true
-
       - name: Run Nix flake checks
         run: nix flake check --print-build-logs