Add artifact upload to tests

Author: gleachkr

.github/workflows/tests.yml

@@ -15,3 +15,9 @@ jobs:
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - run: nix build .#tests
+      - run: mkdir cozy-traces
+      - run: cp result/*.json cozy-traces
+      - uses: actions/upload-artifact@v4
+        with:
+          name: cozy-traces
+          path: cozy-traces/