Fix giomodule.cache being wrongly considered stale

In ostree based systems, such as flatpak and fedora silverblue, the time of modification of every system file is epoch 0, including giomodule.cache, which means that every module is loaded and unloaded every time.

The solution is to use the change time of the file as well. In a typical system, it is equal to the mtime, and in an ostree based system, since the directory is mounted as read-only, the user cannot add a module and we must assume that the cache file corresponds to the modules.

Fixes #2127 (closed)