ue4: store the Mono registry hive in a writable location

Fixes the following error:

    021.05.05-02.07.28:856][303]ERROR: Unhandled exception: System.
    Reflection.TargetInvocationException: Exception has been thrown by
    the target of an invocation. ---> System.Security.SecurityException:
    No access to the given key ---> System.UnauthorizedAccessException:
    Access to the path "/nix/store/nl6mp78dwvyynq1mpmrzfbd7bxy90f1f-ue4-
    4.26.2/share/UnrealEngine/Engine/Build/BatchFiles/Linux/../../../
    Binaries/ThirdParty/Mono/Linux/etc/mono/4.5/../registry/
    LocalMachine" is denied.

pkgs/games/ue4/default.nix

@@ -91,6 +91,9 @@ stdenv.mkDerivation rec {
     # Plugins refer to `.so` files in the main executable directory
     export LD_LIBRARY_PATH="\$sharedir/Engine/Binaries/Linux:\$LD_LIBRARY_PATH"
 
+    # Piggyback on the engine user-local directory
+    export MONO_REGISTRY_PATH="\$HOME/.config/Epic/UnrealEngine/4.26/MonoRegistry"
+
     exec ./UE4Editor "\$@"
     EOF
     chmod +x $out/bin/UE4Editor