Fix runtime.exs when DNS is nil

[conectado]

By using get_env instead of fetch_env when setting wireguard_dns if WIREGUARD_DNS is not set we will get nil instead of an error.

Using this, instead of the server erroring when setting:

default['firezone']['wireguard']['dns'] = nil

in /etc/firezone/firezone.rb.

The server keeps working and when creating a device now we see:

And the generated config is:

[Interface]
PrivateKey = dM/Ypyj/tGe6wgMFcqz/x/gFCQbRw7EljRIH+DEpJSU=
Address = 10.3.2.3/32,fd00::3:2:3/128
MTU = 1420


[Peer]
PresharedKey = Ex8+nbN/7sfYvvBMNg0hQVTNyVYkZsUfYGSIl7T1O18=
PublicKey = aTiAuuEeHlQzMle1pdrYBlXnFO2nBpFe+mko0YUbIhw=
AllowedIPs = 0.0.0.0/0, ::/0
Endpoint = 190.19.29.102:51820
PersistentKeepalive = 0

Note that no dns is added to the config.

This fix was tested on Ubuntu 20.04 in a production enviroment.

Fixes #670