nixos/usbguard: don't use path literal for pure evaluation

PR#256295 reintroduced ruleFile option, but set the default as a path
literal, which was a "string path" previously. This breaks evaluation
for being impure:

error: access to absolute path '/var/lib/usbguard/rules.conf' is forbidden in pure eval mode (use '--impure' to override)

Changed files
+2 -2
nixos
modules
services
security
+2 -2
nixos/modules/services/security/usbguard.nix
···
ruleFile = mkOption {
type = types.nullOr types.path;
-
default = /var/lib/usbguard/rules.conf;
-
example = /run/secrets/usbguard-rules;
+
default = "/var/lib/usbguard/rules.conf";
+
example = "/run/secrets/usbguard-rules";
description = lib.mdDoc ''
This tells the USBGuard daemon which file to load as policy rule set.