Previously, this option was supposed to be a file of the form
`CLOUDFLARE_API_TOKEN=...`, which has a few problems:
- That's not an api token. It's an env file fit for passing to systemd's
`EnvironmentFile` option. The user could typo the variable name, or
intentionally/unintentionally include unrelated environment variables.
- It's not how secret files usually work in NixOS. Secret files are
usually just the secret, and don't leak details about how the secret
is passed to the service.
- This increases friction for people switching between cloudflare dyndns
services, such as `services.cloudflare-dyndns` and
`services.cfdyndns`, which both have a `apiToken` option, but (before
this change) with different semantics.
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