Merge pull request #246962 from khaneliman/alejandra

alejandra: add meta.mainProgram

pkgs/tools/nix/alejandra/default.nix

···
     changelog = "https://github.com/kamadorueda/alejandra/blob/${version}/CHANGELOG.md";
     license = licenses.unlicense;
     maintainers = with maintainers; [ _0x4A6F kamadorueda sciencentistguy ];
+    mainProgram = "alejandra";
   };
 }