Add CentOS SCL repo so we can get Cargo and friends on CI

[avsm]

See https://www.softwarecollections.org/en/scls/rhscl/rust-toolset-7/
and ocaml/opam-repository#17827

[kit-ty-kate]

The rust-toolset-7 package sets does not seem add the binaries as part of the $PATH anyway. I doubt this is useful.
Though if that were, I think this would be better off in opam-depexts the same way epel-release is done than here.