nixos/redshift: add executable option

Added ability to configure the executable that the redshift service
uses.

nixos/modules/services/x11/redshift.nix

@@ -82,6 +82,15 @@ in {
       '';
     };
 
+    executable = mkOption {
+      type = types.str;
+      default = "/bin/redshift";
+      example = "/bin/redshift-gtk";
+      description = ''
+        Redshift executable to use within the package.
+      '';
+    };
+
     extraOptions = mkOption {
       type = types.listOf types.str;
       default = [];
@@ -114,7 +123,7 @@ in {
       partOf = [ "graphical-session.target" ];
       serviceConfig = {
         ExecStart = ''
-          ${cfg.package}/bin/redshift \
+          ${cfg.package}${cfg.executable} \
             -l ${providerString} \
             -t ${toString cfg.temperature.day}:${toString cfg.temperature.night} \
             -b ${toString cfg.brightness.day}:${toString cfg.brightness.night} \