temporary stop of nix CI

.github/workflows/nix.yml

@@ -1,14 +0,0 @@
-name: "Nix CI"
-on:
-  pull_request:
-  push:
-jobs:
-  tests:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@v25
-      with:
-        nix_path: nixpkgs=channel:nixos-unstable
-    - uses: DeterminateSystems/magic-nix-cache-action@v2
-    - run: nix-build