Previously accessible archives are dropped when creating a new ocaml/opam:archive

[kit-ty-kate]

Example:

• archive a is accessible
• ocaml/opam:archive is created and downloads it
• archive a becomes inaccessible
• the next version of ocaml/opam:archive is created without archive a as it 404s

ocaml/opam:archive is used to create opam.ocaml.org: https://github.com/ocaml-opam/opam2web/blob/5d36992e711c0700b2f217aab69ab21e690e2a42/Dockerfile#L52C1-L52C90 and it is very annoying that archive can disappear from one day to the next