Add support for a wildcard value in x-ci-accept-failures to ignore failures on all platforms

[shonfeder]

There are some packages that require configuration of a system beyond opam's (current) capabilities (See, e.g., ocaml/opam-repository#26651 (comment)). It would be unwieldy and brittle to have to list all the platforms it fails on. Instead, let's improve the logic for testing this so that we can test against a regexp (or quivalent). This would make it easy and concise to ignore a class of OS's or all platforms (as needed in the linked case).