Currently if a timezone was selected explicitly, the service will
silently override the value, essentially ignoring what is meant to be a
a deliberate choice of option. This may cause confusion as to why the
option is not doing anything when this service is enabled, particularly
in more complex set-ups after some time.
This will simply make the choice deliberate from the user's part, either
by having to remove the option or lowering its priority as a recognition
that it may be ignored.
This change was inspired by the `services.tzupdate` module, which does
the same.
[1]: <https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/misc/tzupdate.nix#L24>
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