Set the priority of user-set archive-mirrors higher than the repositories'

[kit-ty-kate]

Fixes #4411

I can't find any good reason someone would have a custom cache but want the repository one (even a custom local one) to be taken first.

The one annoying issue with this is that this changes behaviour based on the version of opam used by the user, however we can always announce it before the release (or rather before the change in opam-repository), so it should be ok.

[dra27]

The reasoning for the original behaviour is mentioned in #4411 (comment). Discussing this morning, we think the only downside is the roundtrip for a 404 when trying a global cache which doesn't have a file where the repo cache then does (so the downside is small).

Sadly archive-mirrors isn't itself filtered, but the (negative) feedback when archive-mirrors was added to git+https://github.com/ocaml/opam-repository.git in ocaml/opam-repository#17355 (and was reverted in ocaml/opam-repository#17552) was for CI systems, which should more readily be in a position to update to the required version of opam if the change causes problems. I agree that an announcement is probably sufficient on the opam-repository PR which makes the change.

[kit-ty-kate]

The one annoying issue with this is that this changes behaviour based on the version of opam used by the user, however we can always announce it before the release (or rather before the change in opam-repository), so it should be ok.

Actually there is a way to do it (or not) based on the opam version: ocaml/opam-repository#20130
So this isn’t an issue anymore.

[kit-ty-kate]

@AltGr approved via DM.