Test HTTP backend in CI

[dra27]

#4265 fixes the SHA of opam-repository which at present moves all testing to the Git backend. Given that the http backend is the default, we should be testing it somewhere!

@rjbou suggests having one of the mirror servers @AltGr has been working on be capable of serving index.tar.gz for a specific SHA, so that we can do, say, opam init https://mirror.opam.ocaml.org/8c45759d4.