GHA/windows: re-add GnuTLS for vcpkg, improve perf by building examples less

GHA/windows: re-add GnuTLS for vcpkg, improve perf by building examples less

The GnuTLS MSVC/vcpkg build doesn't actually work on Windows. Let's
restore the build itself, to keep it fit for more testing. With disabled
tests (and examples) to keep it fast and not add to flakiness.

Also:
- enable GnuTLS in the MultiSSL job.
- limit building examples to one normal and one UWP job. It saves
  6 x 1-1.5 minutes. Coverage remains the same, because example builds
  only depend on the toolchain / target, not on the actual features
  (except IPv6, but that's enabled for all.)

Closes #16623