reverting findlib url to download.camlcity.org, and specify a mirror

[gerdstolpmann]

#10383 is a bad idea because the gitlab VM runs on the same machine, and is also not failsafe. I remembered that there is a mirror of download.camlcity.org, and provided that opam is clever enough to pick the mirror in case of a failure, this should be the better solution.

[avsm]

Thanks @gerdstolpmann -- opam2 is better in this regard.

[camelus]

✅ No new or changed opam files 31a9657

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✅ Installability check (7503 → 7503)

[yallop]

Thanks!