… the CI.
There are 2 finale steps before finale binary distribution on Windows.
We must compile the GSettings XML schema files and register GdkPixbuf
loaders (for file format support in the GUI).

I used to provide a wrapper to be run inside Windows before first GIMP
run. Never did I realize that I can compile the distributed GSettings
schemas with the native `glib-compile-schemas` (works fine in my tests).
As for the GdkPixbuf loaders, we inspect DLL libraries, hence we do
require the target `gdk-pixbuf-query-loaders` which is unfortunately a
Windows executable. Yet it seems to work fine with Wine, so let's be
done with it in the CI instead of requiring manual steps from testers of
the CI builds. Then a few `sed` calls are enough to make the path in the
produced text file relative instead of absolute (which works fine, again
in my tests at least).

This means that I don't have to distribute the 2 binaries and the DLLs
they depend on anymore. Moreover let's remove the wrapper (but still
generate one which just calls GIMP so that we call it from the tree
root, where it's much less messy).

Note: I failed to install wine32 (32-bit Wine) on the Gitlab runner.
After following all instructions, I encountered weird errors. So
instead, I just make the win32-nightly job depend on win64-nightly and
copy `loaders.cache` from one to another, as it is a
platform-independent text file (as long as we provide the same GdkPixbuf
loaders on both of course, which we do).