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

.github/workflows/windows.yml

@@ -731,7 +731,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - name: 'openssl'
+          - name: 'openssl +examples'
             install: 'brotli zlib zstd        nghttp2 nghttp3 openssl libssh2'
             arch: 'x64'
             plat: 'uwp'
@@ -753,13 +753,13 @@ jobs:
               -DCURL_USE_GSASL=ON -DENABLE_ARES=ON -DCURL_USE_LIBUV=ON -DCURL_USE_GSSAPI=ON
 
           - name: 'schannel MultiSSL U'
-            install: 'brotli zlib zstd libpsl nghttp2 libssh2[core,zlib] pkgconf gsasl openssl mbedtls wolfssl'
+            install: 'brotli zlib zstd libpsl nghttp2 libssh2[core,zlib] pkgconf gsasl shiftmedia-libgnutls openssl mbedtls wolfssl'
             arch: 'x64'
             plat: 'windows'
             type: 'Debug'
             config: >-
               -DCURL_USE_LIBSSH2=ON
-              -DCURL_USE_SCHANNEL=ON  -DCURL_USE_OPENSSL=ON -DCURL_USE_MBEDTLS=ON -DCURL_USE_WOLFSSL=ON -DCURL_DEFAULT_SSL_BACKEND=schannel
+              -DCURL_USE_SCHANNEL=ON -DCURL_USE_GNUTLS=ON -DCURL_USE_OPENSSL=ON -DCURL_USE_MBEDTLS=ON -DCURL_USE_WOLFSSL=ON -DCURL_DEFAULT_SSL_BACKEND=schannel
               -DCURL_USE_GSASL=ON -DUSE_WIN32_IDN=ON -DENABLE_UNICODE=ON -DUSE_SSLS_EXPORT=ON
 
           - name: 'libressl'
@@ -782,7 +782,7 @@ jobs:
               -DCURL_USE_SCHANNEL=OFF -DCURL_USE_OPENSSL=ON
               -DUSE_ECH=ON
 
-          - name: 'wolfssl'
+          - name: 'wolfssl +examples'
             install: 'brotli zlib zstd libpsl nghttp2 wolfssl libssh2 pkgconf gsasl ngtcp2[wolfssl] nghttp3'
             arch: 'x64'
             plat: 'windows'
@@ -793,6 +793,19 @@ jobs:
               -DCURL_USE_GSASL=ON
               -DUSE_ECH=ON
 
+          - name: 'gnutls'
+            install: 'brotli zlib zstd libpsl nghttp2 shiftmedia-libgnutls libssh2 pkgconf gsasl ngtcp2[gnutls] nghttp3'
+            arch: 'x64'
+            plat: 'windows'
+            type: 'Debug'
+            # GnuTLS is not fully functional on Windows, so skip the tests
+            # https://github.com/ShiftMediaProject/gnutls/issues/23
+            tflags: 'skiprun'
+            config: >-
+              -DCURL_USE_LIBSSH2=ON
+              -DCURL_USE_SCHANNEL=OFF -DCURL_USE_GNUTLS=ON -DUSE_NGTCP2=ON
+              -DCURL_USE_GSASL=ON
+
           - name: 'mbedtls'
             install: 'brotli zlib zstd libpsl nghttp2 mbedtls libssh pkgconf gsasl'
             arch: 'x64'
@@ -901,11 +914,7 @@ jobs:
         if: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' }}
         timeout-minutes: 5
         run: |
-          # GnuTLS is not fully functional on Windows, so skip the tests
-          # https://github.com/ShiftMediaProject/gnutls/issues/23
-          if [[ '${{ matrix.name }}' != *'gnutls'* ]]; then
-            /c/ProgramData/chocolatey/choco.exe install --yes --no-progress --limit-output --timeout 180 --force stunnel openssh || true
-          fi
+          /c/ProgramData/chocolatey/choco.exe install --yes --no-progress --limit-output --timeout 180 --force stunnel openssh || true
           python3 -m pip --disable-pip-version-check --no-input --no-cache-dir install --progress-bar off --prefer-binary impacket
 
       - name: 'downgrade msys2-runtime'
@@ -930,6 +939,7 @@ jobs:
 
       - name: 'build examples'
         timeout-minutes: 5
+        if: ${{ contains(matrix.name, '+examples') }}
         run: |
           PATH="/c/msys64/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target curl-examples