isl: isl.gforge.inria.fr has been taken offline
https://issues.guix.gnu.org/42162
https://github.com/dockcross/dockcross/issues/606
https://groups.google.com/g/isl-development/c/JGaMo2VUu_8
https://giters.com/coq/opam-coq-archive/issues/1298?amp=1
Signed-off-by: Arthur Gautier <baloo@superbaloo.net>
(cherry picked from commit 53a60ad361e020275b3f50c68a7f17411e5f8fd2)
