Harden Windows build setup

[eransha-salvador]

Summary

• Prefer VS 2017/2019/2022 over newer installs (e.g. VS 2026) so hosts with a newer VS alongside VS 2022 still build on v143. Pins VCPKG_VISUAL_STUDIO_PATH / VCPKG_PLATFORM_TOOLSET=v143 so vcpkg's CMake matches the .vcxproj toolset.
• Fall back to the repo root when $env:WORKSPACE is unset so OpenSSH-build.ps1 is runnable outside CI.
• Document Windows prerequisites in README.txt.

Fixes PowerShell/Win32-OpenSSH#2434

Test plan

• Run Start-OpenSSHBuild on a host with both VS 2022 and VS 2026 installed; verify v143 toolset is selected and build succeeds.
• Run OpenSSH-build.ps1 locally (no WORKSPACE set) and verify artifacts land in the repo root.
• Fresh clone: follow README.txt and build end-to-end.

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[tgauth]

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