Check `x-ci-accept-failures` field format

[sim642]

There should be a CI check for the x-ci-accept-failures field because there packages which use formats that aren't intended and silently just don't work.

AFAICS, under x-ci-accept-failures we list operating systems from the first list here: https://github.com/ocurrent/opam-repo-ci/blob/master/doc/platforms.md
I don't know how if it is possibly to quantify that to debian-13 only on arm64 though... 🤔

Hmm, that seems to be the case indeed, looking at opam-repo-ci sources.

Oddly enough, I found some packages that have tried looking for other parts of the variant string though (and also has extra brackets!):
https://github.com/ocaml/opam-repository/blob/5f4fdc0704abf51f92d9cde86eaaeb31f259ae59/packages/echo/echo.0.0.5/opam#L16-L19
https://github.com/ocaml/opam-repository/blob/5f4fdc0704abf51f92d9cde86eaaeb31f259ae59/packages/frama-c/frama-c.24.0/opam#L139-L142
And even one which uses a formula there:
https://github.com/ocaml/opam-repository/blob/5f4fdc0704abf51f92d9cde86eaaeb31f259ae59/packages/conf-rust-llvm/conf-rust-llvm.1/opam#L25
I suspect that one accidentally even works because opam-repo-ci just greps the value on the shell.

Originally posted by @sim642 in ocaml/opam-repository#28478 (comment)