Add a "trust cache" repository option

[AltGr]

No description provided.

[AltGr]

This is an attempted quickfix for the automated repo cache at https://gitlab.ocamlpro.com/OCamlPro/opam-repository/-/tree/cached : the repository fetched from CI has git LFS handles for the cached files, but not the full files, so their hashes can't be recomputed.

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(it worked)

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The OCamlPro repo cache (https://gitlab.ocamlpro.com/OCamlPro/opam-repository/-/tree/cached) still needs this feature

[kit-ty-kate]

Sorry for the lack of feedback. We had discussed it in a meeting a while back and forgot to write the conclusion here. I personally think having this environment variable available for everyone is too dangerous. I think having an argument limited to opam admin cache would be just fine although i'm not sure what's the best API to have for this, maybe just a new ?unsafe_trust_cache:bool argument to fetch_from_cache would be enough