virtualbox: install UnattendedTemplates

Fix automated/unattended guest installs by having the missing templates
where virtualbox expects them.

Fixes #247807.

pkgs/applications/virtualization/virtualbox/default.nix

···
     ''}
 
     cp -rv out/linux.*/${buildType}/bin/src "$modsrc"
+
+    mkdir -p "$out/share/virtualbox"
+    cp -rv src/VBox/Main/UnattendedTemplates "$out/share/virtualbox"
   '';
 
   preFixup = optionalString (!headless) ''