Upload CI Cargo.lock for reproducing failures

Author: dtolnay

.github/workflows/ci.yml

@@ -38,6 +38,11 @@ jobs:
       - run: cargo check --no-default-features
       - run: cargo check --features serde
       - run: cargo check --no-default-features --features serde
+      - uses: actions/upload-artifact@v4
+        if: matrix.rust == 'nightly' && always()
+        with:
+          name: Cargo.lock
+          path: Cargo.lock
 
   node:
     name: Node