Resets hostname to the one provided by the DHCP server

I’ve seen (once, for now) a Tails ISO built from current experimental have its hostname reset to the one provided by the DHCP server:

$ hostname
whatever

We don’t try that hard to avoid leaking the hostname, so this is an important issue.

Parent Task: #7688 (closed)

Original created by @intrigeri on 7769 (Redmine)