Upload CI Cargo.lock for reproducing failures

Author: dtolnay

.github/workflows/ci.yml

@@ -37,6 +37,11 @@ jobs:
       - run: cargo check
       - run: cargo test --workspace
         if: matrix.rust != '1.56.0'
+      - uses: actions/upload-artifact@v4
+        if: matrix.rust == 'nightly' && always()
+        with:
+          name: Cargo.lock
+          path: Cargo.lock
 
   doc:
     name: Documentation