openra: patch to build with .NET 8 (from EOL .NET 6) (#490249)

pkgs/games/openra/build-engine.nix

@@ -35,6 +35,12 @@ buildDotnetModule {
 
   nugetDeps = engine.deps;
 
+  # Retarget from net6.0 to net8.0 (net6.0 is EOL)
+  postPatch = ''
+    substituteInPlace Directory.Build.props \
+      --replace-fail ">net6.0<" ">net8.0<"
+  '';
+
   useAppHost = false;
 
   dotnetFlags = [ "-p:Version=0.0.0.0" ]; # otherwise dotnet build complains, version is saved in VERSION file anyway

pkgs/games/openra/engines/release/default.nix

@@ -5,5 +5,5 @@ buildOpenRAEngine {
   version = "20250330";
   hash = "sha256-chWkzn/NLZh2gOua9kE0ubRGjGCC0LvtZSWHBgXKqHw=";
   deps = ./deps.json;
-  dotnet-sdk = dotnetCorePackages.sdk_6_0-bin;
+  dotnet-sdk = dotnetCorePackages.sdk_8_0;
 }