GHA/windows: install OpenSSH-Windows manually for transparency

To have the current latest version, and to avoid the stale, misleading
versions installed by Chocolatey. It also installs transparently, faster,
and making the source of the binaries clear. Install on drive `D:` for
best performance.

After much detective work it turns out that the OpenSSH Windows versions
installed by Chocolatey aren't what they seem:

- The latest pre-release named 9.5.0-beta20240403:
  https://community.chocolatey.org/packages/openssh/9.5.0-beta20240403
  is in reality 8.6.0.0p1-Beta from 2021-05-27:
  https://github.com/PowerShell/Win32-OpenSSH/releases/download/V8.6.0.0p1-Beta/OpenSSH-Win64.zip

- The latest "stable" version 8.0.0.1 is in reality:
  https://community.chocolatey.org/packages/openssh/8.0.0.1
  is in reality 8.0.0.0p1-Beta:
  https://github.com/PowerShell/Win32-OpenSSH/releases/download/v8.0.0.0p1-Beta/OpenSSH-Win64.zip

Ref: https://github.com/curl/curl/pull/16803#issuecomment-2746365654
Follow-up to 67a7775d12 #16704
Follow-up to 0ec72c1ef8 #16672

Closes #16811

.github/workflows/windows.yml

@@ -738,6 +738,7 @@ jobs:
       run:
         shell: msys2 {0}
     env:
+      openssh_windows-version: 'v9.8.1.0p1-Preview'
       VCPKG_BINARY_SOURCES: 'clear;x-gha,readwrite'
       VCPKG_DISABLE_METRICS: '1'
     strategy:
@@ -927,10 +928,13 @@ jobs:
           if [ '${{ matrix.openssh }}' = '' ]; then  # MSYS2 openssh
             /usr/bin/pacman --noconfirm --noprogressbar --sync --needed openssh
           else  # OpenSSH-Windows
-            [ '${{ matrix.openssh }}' = 'OpenSSH-Windows-Pre' ] && chocopkg+=' --prerelease'
-            chocopkg+=' openssh'
+            cd /d || exit 1
+            curl --disable --fail --silent --show-error --connect-timeout 15 --max-time 60 --retry 3 \
+              --location 'https://github.com/PowerShell/Win32-OpenSSH/releases/download/${{ env.openssh_windows-version }}/OpenSSH-Win64.zip' --output bin.zip
+            unzip bin.zip
+            rm -f bin.zip
           fi
-          /c/ProgramData/chocolatey/choco.exe install --yes --no-progress --limit-output --timeout 180 --force ${chocopkg} stunnel || true
+          /c/ProgramData/chocolatey/choco.exe install --yes --no-progress --limit-output --timeout 180 --force stunnel || true
           python3 -m pip --disable-pip-version-check --no-input --no-cache-dir install --progress-bar off --prefer-binary impacket
 
       - name: 'downgrade msys2-runtime'
@@ -957,7 +961,7 @@ jobs:
             else
               TFLAGS+=' ~3022'  # 'SCP correct sha256 host key' SCP, server sha256 key check
             fi
-            PATH="$PATH:/c/Program Files/OpenSSH-Win64"
+            PATH="/d/OpenSSH-Win64:$PATH"
           fi
           PATH="$PWD/bld/lib/${{ matrix.type }}:$PATH:/c/Program Files (x86)/stunnel/bin"
           cmake --build bld --config '${{ matrix.type }}' --target test-ci