fixup! MB 112: Updater customization for Mullvad Browser

MB 200: Enable system installs for Mullvad Browser

Customize the post update executable name, to avoid any confusion with
Mozilla's helper.exe, since we intend using a much simpler post update
binary.

browser/installer/windows/nsis/updater_append.ini

@@ -7,6 +7,8 @@
 [PostUpdateWin]
 ; ExeRelPath is the path to the PostUpdateWin executable relative to the
 ; application executable.
-ExeRelPath=uninstall\helper.exe
+ExeRelPath=postupdate.exe
 ; ExeArg is the argument to pass to the PostUpdateWin exe
+; We do not need any argument, but an empty string here will make updater.exe
+; not run this step.
 ExeArg=/PostUpdate