Enhance build for Windows

[eransha-salvador]

Several friction points when building OpenSSH on Windows with current VS releases:

1. Start-OpenSSHBuild calls vswhere and takes the newest install it returns.
   On machines that also have VS 2026 (v145 toolset) installed alongside
   VS 2022 (v143), the downstream Select-String version checks miss, fall
   through to the VS 2015 branch, and dereference a null $env:VS140COMNTOOLS.

2. vcpkg's CMake can pick a different toolset than the .vcxproj files
   (which hard-code v143). When a newer VS is selected, builds fail with
   MSB8040 (Spectre-mitigated libs missing for v145).

3. OpenSSH-build.ps1 -destination defaults to $env:WORKSPACE. Outside
   CI that variable is unset, so running the script locally errors before
   it starts.

4. README.txt lacks current Windows prerequisites: VS 2022 Desktop C++
   workload, v143 Spectre-mitigated libs, Git, the one-time vcpkg
   bootstrap/integrate step, and the need for elevated PowerShell.

All four are build-environment issues, independent of runtime behavior.