Upload CI Cargo.lock for reproducing failures

Author: dtolnay

.github/workflows/ci.yml

@@ -56,6 +56,11 @@ jobs:
         if: matrix.rust == 'nightly'
       - run: cargo test
       - run: cargo check --benches
+      - uses: actions/upload-artifact@v4
+        if: matrix.rust == 'nightly' && always()
+        with:
+          name: Cargo.lock
+          path: Cargo.lock
 
   msrv:
     name: Rust 1.31.0