nixos/lemmy: only use env var when instructed

Lemmy checks the environment variable before the configuration file;
i.e. if the file is used to configure the database but the environment
variable is set to anything, the connection will fail because it'll
ignore the file. This was the previous behavior.

Now, the environment variable will be unset unless the user explicitly
chooses to set it, which makes the file-based configuration function
correctly. It's also possible to manually set the environment variable,
which has the major advantage of working around [this issue][0], which
prevents certain setups from working.

[0]: https://github.com/LemmyNet/lemmy/issues/2945

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