dbus: Add NSS modules path to dbus system bus service

 DBus seems to resolve user IDs directly via glibc, circumventing nscd. In more
 advanced setups this leads to user's coming from LDAP or SSSD not being
 resolved by the dbus system bus daemon. The effect for such users is, that all
 access to the system bus (e.g. busctl or nmcli) is denied.

 Adding the respective NSS modules to the service's environment solves the issue
 the same way it does for nscd.