flake.nix: check build

flake.nix

@@ -34,7 +34,11 @@
         program = toString (
           pkgs.writeShellScript
           "build"
-          "rm -rf dist/* && ${python}/bin/python -m build"
+          ''
+            rm -rfv dist/*
+            ${python}/bin/python -m build
+            ${python}/bin/python -m twine check dist/*
+          ''
         );
       };