Fall back to an older opam release when the latest lacks binaries

[smorimoto]

Problem

opam 2.5.1 was released with only a source tarball (opam-full-2.5.1.tar.gz) and no pre-built binaries. The current logic selects the latest semver-matching release and then looks for a binary asset for the runner's platform and architecture. When the latest release has no binaries, the action fails immediately with:

Failed to find opam binary for 'linux' and 'x86_64'. Please check if this combination is supported by opam.

This broke CI for all users on the default (stable) configuration. See the failing run: https://github.com/ocaml/setup-ocaml/actions/runs/24321628257

Solution

Instead of checking only the single latest release, iterate through all semver-matched releases (sorted newest-first) and return the first one that contains a binary asset for the current platform and architecture. If no release has a suitable binary, the same error is raised as before.

This means opam 2.5.1 (source-only) is silently skipped and opam 2.5.0 (which has full binary assets) is used instead.

Verification

• npm run build — passes
• npm run typecheck — passes
• npm run lint — passes (the only flagged file is .claude/settings.local.json, unrelated to this change)
• The fix can be verified end-to-end by running the action on any platform; it should now resolve to opam 2.5.0 instead of erroring out on 2.5.1