It enables passing a sops-nix secret as a `settingsFile`
@see https://github.com/Mic92/sops-nix/issues/198.
By default sops-nix secrets are accessible by only root. We can change owner to another user, but the xray service is defined with `dynamicUser=true`, which means, there is no user in the compile time.
Systemd `loadCredential` passes the secret file to the service, which is exactly what we need here.
these changes were generated with nixq 0.0.2, by running
nixq ">> lib.mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
nixq ">> mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
nixq ">> Inherit >> mdDoc[remove]" --batchmode nixos/**.nix
two mentions of the mdDoc function remain in nixos/, both of which
are inside of comments.
Since lib.mdDoc is already defined as just id, this commit is a no-op as
far as Nix (and the built manual) is concerned.
