Fix translation of file:// URIs into paths

Fix translation of file:// URIs into paths

Currently if a file path contains a special character such as '\', and
that character is encoded into a file:// URI that is passed to
ostree_repo_pull_with_options(), the percent encoding will remain in the
path passed to g_file_new() (in the case of backslash %5C) and the pull
will then fail with a file not found error. This is an important edge
case to handle because by default on many Linux distributions a
filesystem with no label is mounted at a path based on its UUID, and
this is then passed to systemd-escape by Flatpak (when
--enable-auto-sideloading was used at compile time) to create a symbolic
link such as this which contains backslashes:

$ ls -l /run/flatpak/sideload-repos/
total 0
lrwxrwxrwx 1 mwleeds mwleeds 55 Mar  9 14:21
'automount-run-media-mwleeds-29419e8f\x2dc680\x2d4e95\x2d9a31\x2d2cc907d421cb'
-> /run/media/mwleeds/29419e8f-c680-4e95-9a31-2cc907d421cb

And Flatpak then passes libostree a file:// URI containing that path, to
implement sideloading (pulling content from the USB drive).

This results in an error like:

Error: While pulling app/org.videolan.VLC/x86_64/stable from remote
flathub:
/run/flatpak/sideload-repos/automount-run-media-mwleeds-29419e8f%5Cx2dc680%5Cx2d4e95%5Cx2d9a31%5Cx2d2cc907d421cb/.ostree/repo:
opendir(/run/flatpak/sideload-repos/automount-run-media-mwleeds-29419e8f%5Cx2dc680%5Cx2d4e95%5Cx2d9a31%5Cx2d2cc907d421cb/.ostree/repo):
No such file or directory

This patch avoids such errors by using g_file_new_for_uri() instead of
g_file_new_for_path(), so that GLib handles the %-decoding for us.

Bug report by user:
https://community.endlessos.com/t/can-not-install-vlc-from-usb-drive-3-9-3/16353