Upload CI Cargo.lock for reproducing failures

Author: dtolnay

.github/workflows/ci.yml

@@ -26,6 +26,11 @@ jobs:
           toolchain: ${{matrix.rust}}
       - run: cargo test
         working-directory: _skeptic
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: Cargo.lock
+          path: _skeptic/Cargo.lock
 
   outdated:
     name: Outdated