Windows GHA CI green on invalid opam file

[jmid]

Spotted on #29486:

When encountering an invalid opam file, the Windows GHA workflows print a warning as expected,
but still complete with a green light.

https://github.com/ocaml/opam-repository/actions/runs/22796601588/job/66132129043?pr=29486

Run $failed = $false

<><> Updating package repositories ><><><><><><><><><><><><><><><><><><><><><><>
[default] synchronised from file://D:/a/opam-repository/opam-repository
Warning:  Could not read file D:\opamroot\repo\default\packages\biotk\biotk.0.4\opam. skipping:
          At D:\opamroot\repo\default\packages\biotk\biotk.0.4\opam:24:13-24:14::
          '.' is not a valid token
Now run 'opam upgrade' to apply any package updates.
Testing biotk.0.4
  [ERROR] Package biotk has no version 0.4.
biotk.0.4 is not installable. Skip.

I think we just need to check the exit code of the opam update too.