mono-env_pkg_setup sets MONO_REGISTRY_PATH to avoid sandbox errors from packages that would otherwise try to write to /etc/mono/registry. The dotnet eclass inherits mono-env but dotnet_pkg_setup doesn't call mono-env_pkg_setup, so packages like dev-dotnet/gtk-sharp fail to build due to sandbox violations.

See:

• https://bugs.gentoo.org/659422
• https://github.com/gentoo/dotnet/commit/bcf8f3e6e0be19140ad7951e6f51db93a980aecc