fetcher: Use 0666 (-umask) for temporary files

fetcher: Use 0666 (-umask) for temporary files

There's no reason to keep them hidden.  I have a hard policy that
OSTree should *not* be used to carry secrets.  Things like host ssh
private keys should be set up out of band by an OS-external
configuration mechanism such as kickstart, cloud-init, etc.

We also assume that hiding binaries is not very useful as most
attackers would be able to find them on the Internet or (for
subscribed content) acting as a customer.

This fixes a bug with mirroring because we changed to take the
unmodified upstream objects rather than uncompress <-> recompress.

https://bugzilla.gnome.org/show_bug.cgi?id=748959