What does this PR aim to accomplish?:

During fresh installation, we ask the user which interface should be used for Pi-hole. This populates PIHOLE_INTERFACE, but we never used the var for anything. This PR now sets the this as dns.interface which can be used by FTL if necessary.
Reported on discourse: https://discourse.pi-hole.net/t/problems-setting-up-new-v6-installation/79633

P.S. By default, the listening mode is local which means dns.interface is unused. But the web interface allows with a single click to switch to a different listening mode which will use the specified interface.

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