lib/repo: Don't copy xattrs when manipulating the GPG keyring

lib/repo: Don't copy xattrs when manipulating the GPG keyring

Copying xattrs when manipulating the GPG keyring for a repository
causes errors when the underlying filesystem doesn't support writing
xattrs - overlayfs is a common example. It also causes the selinux
attributes of the keyring files to be copied from the temporary
location instead of properly inherited from the destination directory
(ending up, for example, as unconfined_u:object_r:user_tmp_t:s0, rather
than unconfined_u:object_r:data_home_t:s0)

Closes: #910
Approved by: cgwalters