On NixOS packages are installed in separate prefixes.
Starting from GLib / gobject-introspection 2.80, GLib introspection data
is provided by GLib itself instead of gobject-introspection. This causes
tests failures on NixOS because env.set() resets the environment and GLib
is missing from GI_TYPELIB_PATH:

gi.RepositoryError: Typelib file for namespace 'Gio', version '2.0' not found

See also:
gtk@29e6cc58