nixos/openarena: fix service's binary name

The binary name was recently changed from openarena-server to oa_ded in
https://github.com/NixOS/nixpkgs/pull/71122 .

That change broke the openarena module and consequently the openarena
test too. This commit fixes both.

As an alternative, we considered reverting the name change in
https://github.com/NixOS/nixpkgs/pull/72824 but we decided oa_ded was
a better name for the binary (it's the name upstream use).