Previously, when merging different definitions of `extraConfig`,
only the top-level attrset was merged.
Example:
The two separate settings
nodes.lnd.extraConfig.Settings.userPersona = "MERCHANT";
nodes.lnd.extraConfig.Settings.logLevel = "DEBUG";
were previously merged into
nodes.lnd.extraConfig.Settings = { logLevel = "DEBUG" };
(The last definition has precedence.)
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