Patch to move from caml.inria.fr to github.com

[mtelvers]

This fix is necessary due to ocaml/ocaml#13237 and allow us to build base images ocurrent/docker-base-images#283 again.

I have tested this patch in the base image builder using this commit and successfully built a base image with opam 2.0, 2.1 and dev.

[mtelvers]

# docker run --rm -it docker.io/ocurrent/opam-staging:ubuntu-24.04-opam-arm64
opam@70d084a3b203:~$ opam-2.0 --version
2.0.10
opam@70d084a3b203:~$ opam-2.1 --version
2.1.6
opam@70d084a3b203:~$ opam-2.2 --version
2.2.0~rc1~dev
opam@70d084a3b203:~$ opam-dev --version
2.2.0~rc1~dev

[shonfeder]

LGTW, fwiw :)

[dra27]

LGTM, thanks. It seems strange that we (opam) use caml.inria.fr when opam-repository's ocaml-base-compiler.4.14.2 uses GitHub already, so I think the consistency is good, regardless of the downtime on caml.inria.fr