pkgs: add explicit 'meta.mainProgram'

Since upstream has decided to deprecate the implied value when using
`lib.getExe` (see [1]), we should set it explicitly.

[1]: https://github.com/NixOS/nixpkgs/pull/246386

pkgs/lohr/default.nix

@@ -16,6 +16,7 @@ rustPlatform.buildRustPackage rec {
     description = "Git mirroring daemon";
     homepage = "https://github.com/alarsyo/lohr";
     license = with licenses; [ mit asl20 ];
+    mainProgram = "lohr";
     maintainers = with maintainers; [ ambroisie ];
     platforms = platforms.unix;
   };