This strong preference fixes a sporadic issue when
concretizing environments with `unify:when_possible`.
In the first round of concretization, it is almost
certain that glibc is installed, and that spec might
provide iconv.
In later rounds using that as a provider might be
preferred, as it leads to less nodes to be "built".
To avoid duplication by default, prefer libiconv
in a stronger way than default preferences.
Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>
c5b227d14c