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.