fix ci and make it reproducible

.github/workflows/test.yml

@@ -7,15 +7,7 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        # when updating channels, also update .mergify.yml
-        channel: [ nixos-unstable, nixos-24.05 ]
     steps:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@V27
-      with:
-        nix_path: nixpkgs=channel:${{ matrix.channel }}
-    - name: Show nixpkgs version
-      run: nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
-    - run: ./tests/run.py
+    - run: nix run ./tests#run .