fix WIX_EXTENSIONS as path (#4171)

packages/w/wixtoolset/xmake.lua

@@ -8,6 +8,11 @@ package("wixtoolset")
     add_versions("5.0.0", "e8243606c71fa5bc2e0eb14d6005f42f1282b61540fb224b0004de563a81f74d")
     add_resources("5.0.0", "ui", "https://www.nuget.org/api/v2/package/WixToolset.UI.wixext/5.0.0/#ui-5.0.0.zip", "fd0ccff8bf56eeb5fe306f3ad09eb74ba9546186f51d9d065f75dfc28310aa9d")
 
+    on_load(function (package)
+        package:addenv("WIX_EXTENSIONS", "bin")
+        package:mark_as_pathenv("WIX_EXTENSIONS")
+    end)
+
     on_install("windows", function (package)
         import("lib.detect.find_file")
         import("lib.detect.find_directory")
@@ -17,8 +22,6 @@ package("wixtoolset")
         local version = package:version():rawstr()
         local ui_folder = path.join(package:installdir("bin"), ".wix", "extensions", "WixToolset.UI.wixext", version)
         os.cp(path.join(package:resourcedir("ui")), ui_folder)
-        package:addenv("WIX_EXTENSIONS", "bin")
-        package:mark_as_pathenv("WIX_EXTENSIONS")
     end)
 
     on_test(function(package)