Upload CI Cargo.lock for reproducing failures

Author: dtolnay

.github/workflows/ci.yml

@@ -77,6 +77,11 @@ jobs:
         run: cargo test
         env:
           RUSTFLAGS: -Z allow-features= --cfg procmacro2_backtrace ${{env.RUSTFLAGS}}
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: Cargo.lock
+          path: Cargo.lock
 
   layout:
     name: Layout