alttpr-opentracker: only enable on x86_64-linux

It was failing to build on aarch64-linux with:

Executing dotnetConfigureHook
Determining projects to restore...
Restored /build/source/OpenTracker.UnitTests/OpenTracker.UnitTests.csproj (in 35.07 sec).
Restored /build/source/OpenTracker.Models/OpenTracker.Models.csproj (in 520 ms).
Restored /build/source/OpenTracker.Utils/OpenTracker.Utils.csproj (in 48 ms).
/build/source/OpenTracker/OpenTracker.csproj : error NU1101: Unable to find package Microsoft.NETCore.App.Host.linux-arm64. No packages exist with this id in source(s): /nix/store/x82zwgmqdbp4xykx1nkrjin8hicm8jld-opentracker-1.8.2-nuget-source/lib [/build/source/OpenTracker.sln]
Failed to restore /build/source/OpenTracker/OpenTracker.csproj (in 12.45 sec).
Restored /build/source/Avalonia.ThemeManager/Avalonia.ThemeManager.csproj (in 56 ms).

pkgs/tools/games/opentracker/default.nix

···
     license = licenses.mit;
     maintainers = [ maintainers.ivar ];
     mainProgram = "OpenTracker";
-    platforms = platforms.linux;
+    platforms = [ "x86_64-linux" ];
   };
 }