GHA/windows: restore a simple vcpkg job (zlib + libssh2)

To keep the basics tested. Building zlib and libssh2 takes <1.5 minutes.

So far it seems the "lost" vcpkg binary cache is not a vcpkg-tool
regression, but a GitHub server-side deprecation combined with lack of
replacement service or lack of vcpkg support for such service.

Ref: https://github.blog/changelog/2025-03-20-notification-of-upcoming-breaking-changes-in-github-actions/
Ref: microsoft/vcpkg#45073

Follow-up to 15fb1dc curl#17069

Closes curl#17086

Author: vszakats