services(cloudflare-dyndns): use new CLOUDFLARE_API_TOKEN_FILE setting

We recently upgraded to cloudflare-dyndns 5.3:
https://github.com/NixOS/nixpkgs/commit/24f9910708d0d38c0e0cd9cc863bde4149b48fb6,
so we can now use this new `CLOUDFLARE_API_TOKEN_FILE` feature that
landed in v5.2: https://github.com/kissgyorgy/cloudflare-dyndns/commit/1d563d27529e5c5373d1b747536ea09fa8366d88

nixos/modules/services/networking/cloudflare-dyndns.nix

@@ -108,13 +108,14 @@ in
               ++ lib.optional cfg.proxied "--proxied";
           in
           ''
-            export CLOUDFLARE_API_TOKEN=$(< "''${CREDENTIALS_DIRECTORY}/apiToken")
+            export CLOUDFLARE_API_TOKEN_FILE=''${CREDENTIALS_DIRECTORY}/apiToken
 
             # Added 2025-03-10: `cfg.apiTokenFile` used to be passed as an
             # `EnvironmentFile` to the service, which required it to be of
             # the form "CLOUDFLARE_API_TOKEN=" rather than just the secret.
             # If we detect this legacy usage, error out.
-            if [[ $CLOUDFLARE_API_TOKEN == CLOUDFLARE_API_TOKEN* ]]; then
+            token=$(< "''${CLOUDFLARE_API_TOKEN_FILE}")
+            if [[ $token == CLOUDFLARE_API_TOKEN* ]]; then
               echo "Error: your api token starts with 'CLOUDFLARE_API_TOKEN='. Remove that, and instead specify just the token." >&2
               exit 1
             fi