nixos/syncthing: respect the dataDir option

The previous -home argument worked as such:
"Set common configuration and data directory. The default configuration directory is $HOME/.config/syncthing (Unix-like), $HOME/Library/Application Support/Syncthing (Mac) and %LOCALAPPDATA%\Syncthing (Windows)"

This resulted in syncthing not respecting different home and data dirs
declared in its config. The default behaviour will remain the same, as
we set the datadir default value to homeDir + .config/syncthing.

nixos/modules/services/networking/syncthing.nix

@@ -666,7 +666,9 @@ in {
             ${cfg.package}/bin/syncthing \
               -no-browser \
               -gui-address=${if isUnixGui then "unix://" else ""}${cfg.guiAddress} \
-              -home=${cfg.configDir} ${escapeShellArgs cfg.extraFlags}
+              -config=${cfg.configDir} \
+              -data=${cfg.dataDir} \
+              ${escapeShellArgs cfg.extraFlags}
           '';
           MemoryDenyWriteExecute = true;
           NoNewPrivileges = true;