nix: move package to nixpkgs

.github/workflows/nix.yml

@@ -1,6 +1,7 @@
-name: "Nix CI"
+name: "CI - Nix"
 
-on: [pull_request, push]
+on:
+  push:
 
 jobs:
   tests:
@@ -11,10 +12,9 @@ jobs:
         os: [ubuntu]
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
-      - run: nix --accept-flake-config build -L
-      - run: nix --accept-flake-config run .#cachix push gepetto $(readlink result)
-        if: github.repository_owner == 'humanoid-path-planner'
-        env:
-          CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
+      - uses: cachix/install-nix-action@v27
+      - uses: cachix/cachix-action@v15
+        with:
+          name: gepetto
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - run: nix build -L