make sure the rsync updater is always executed from its own dir

misc/tools/rsync-updater/update-xonotic.bat

@@ -1,5 +1,7 @@
 @echo off
 
+cd %~dp0
+
 set options=-Prtzil --executability --delete-after --delete-excluded --stats
 
 if exist Xonotic-low goto xonoticlow

misc/tools/rsync-updater/update-xonotic.sh

@@ -1,5 +1,9 @@
 #!/bin/sh
 
+if [ -d "${0%/*}" ]; then
+	cd "${0%/*}"
+fi
+
 if ! which rsync >/dev/null; then
 	echo >&2 "FATAL: rsync not found, please install the rsync package"
 	exit 1