Don't prefix package name with './' if package is in $PKGDEST

Commit ad593b8b introduces
a problem when $PKGDEST is set and the package to transfer
has an absolute pathname. In this case there is no need
to add anything to have rsync work properly.

Signed-off-by: Rémy Oudompheng <remy@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>