VSToolsPath is reliably set by setup-msbuild/VS environment and points
to where VSSDK targets are installed. VsSDKInstall (NuGet package path)
is kept as a fallback. Both are conditional to avoid failures when
neither is set (e.g. during the restore pass).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>