setBackendsWithDefaultOverrideHostIfMissing doesn't actually do anything

[harmony7]

Version

% fastly version
Fastly CLI version v10.19.0 (33a28544)
Built with go version go1.22.12 linux/amd64 (2025-03-09)
Viceroy version: viceroy 0.12.2

What happened

In #843, a feature was added that checks if override_host is defined for each backend defined in local_server, and sets one equal to the hostname specified as the url.

Unfortunately, after this value is set in memory, it is not actually passed into Viceroy (Viceroy only receives the path to fastly.toml), so it does not see the updated value.

The expected behavior is for the override_host value that is set by this feature to reflect in the Viceroy session that is spawned.