openra: add release- prefix to version string (#262324)

Change version string to have prefix `release-`. This fixes "Server is
running an incompatible version.", when trying to play against official
builds of the OpenRA.

Signed-off-by: Mika Tammi <[e-mail]>

pkgs/games/openra/build-engine.nix

···
   dontDotnetFixup = true;
 
   preBuild = ''
-    make VERSION=${version} version
+    make VERSION=${engine.build}-${version} version
   '';
 
   postInstall = ''