tun: explicitly set IPv6 "stable-privacy" addr-gen-mode

If we are going to check the presence of a IPv6 link-local address, we must make sure that the connection uses "stable-privacy" as IPv6 address generation mode; addr-gen-mode "eui64" requires the presence of a MAC address, which is not available on L3 interfaces as tun.

This change is needed Fedora gating, where the environment is initialized with the following configuration, which sets the mode to EUI64 globally:

  [connection.30-cloud-init-ip6-addr-gen-mode]
  ipv6.addr-gen-mode=0

For example, see:

https://bodhi.fedoraproject.org/updates/FEDORA-2024-1cf1775ce8

https://artifacts.dev.testing-farm.io/1d628da4-4521-443f-a864-34ad568a965a/work-tests.ymlzpam9ixu/tests-_trn0ww7/FAIL-report_NetworkManager-ci_Test0128_add_default_tun_device.html