Add `x-ci-failure` on OPAM packages

[dinosaure]

The last release shows to us that Solo5 can not be compiled on CentOS 7. Currently, we added a small metadata on OPAM packages which disallow an installation of Solo5 on CentOS 7 for opam-ci. I think that we should just say that Solo5 is not available on CentOS 7 when the distribution is pretty old and we don't really want to support it.