Fix autoupdate package version nil (#3982)

scripts/autoupdate.lua

@@ -69,8 +69,8 @@ function _update_version(instance, version, shasum)
     os.vexec("git checkout %s", branch)
     local inserted = false
     local scriptfile = path.join(instance:scriptdir(), "xmake.lua")
-    if os.isfile(scriptfile) then
     local version_current
+    if os.isfile(scriptfile) then
         io.gsub(scriptfile, "add_versions%(\"(.-)\",%s+\"(.-)\"%)", function (v, h)
             if not version_current or semver.compare(v, version_current) > 0 then
                 version_current = v