external-appstream: Don't bother about permissions while copying

Don't bother with permissions while copying the external appstream file from user's cache to APPSYSTEM_SYSTEM_DIR. This already has been now taken care of by the change introduced in [1].

This reason this change is required, is because combination of [1] and G_FILE_COPY_TARGET_DEFAULT_PERMS ends up with permissions as:

-rw------- 1 root root

as G_FILE_COPY_TARGET_DEFAULT makes entire "unix::mode" attribute to be skipped while copying.

Since, as per [1], G_FILE_CREATE_PRIVATE will setup appropriate permissions on the destination file itself, avoid injecting the G_FILE_COPY_TARGET_DEFAULT flag in g_file_copy() to complicate the matter.

(Unfortunately, this requires a glib version bump for upstream)

[1] glib!876 (merged)

https://phabricator.endlessm.com/T27828