build-fhs-userenv: Propagate $XAUTHORITY

The `DISPLAY` environment variable is propagated into chroots built with
`buildFHSUserEnv`, but currently the `XAUTHORITY` variable is not. When
the latter is set, its value is usually necessary in order to connect to
the X server identified by the former.

This matters for users running gdm3, for example, who have `XAUTHORITY`
set to something like `/run/user/1000/gdm/Xauthority` instead of the X
default of `~/.Xauthority`, which doesn't exist in that setup.

Fixes #21532.