iconv: add a strong preference on libiconv by default (#50020)

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>

etc/spack/defaults/packages.yaml

@@ -92,6 +92,8 @@ packages:
     buildable: false
   hpcx-mpi:
     buildable: false
+  iconv:
+    prefer: [libiconv]
   mpt:
     buildable: false
   spectrum-mpi: