GHA Windows: add a special case for packages that are not available

Author: kit-ty-kate

.github/workflows/windows.yml

@@ -77,6 +77,7 @@ jobs:
             opam install --confirm-level=unsafe-yes "$pkg"
             switch ($LASTEXITCODE) {
               0 { Break }
+              5 { Write-Host "$pkg is not installable. Skip."; Break } # TODO: Remove when https://github.com/ocaml/opam/issues/6017 is fixed
               20 { Write-Host "$pkg is not installable. Skip."; Break }
               31 { Write-Host "$pkg failed to build."; $failed = $true; Break }
               default { throw "Unexpected error $_" }