Set path to configuration file with a commandline flag

[ghthor]

Please describe the feature you want

I would like to be able to specify the configuration file path on the command line. The current situation where the configuration is loaded from the root of TABBY_ROOT required this odd workaround when creating NixOS package so we could write the configuration file to /etc/tabby and then still access it by copying it over to TABBY_ROOT JIT when the service is launched.

Additional context

https://github.com/NixOS/nixpkgs/pull/292873/files#diff-315156ccea9aff9aeef8e19d7375f4faaf27c61d17972144d9695e7c08201f9eR164

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Please reply with a 👍 if you want this feature.