dotfiles: put vswhere.exe on the user PATH

dotfiles: put vswhere.exe on the user PATH

The Visual Studio installer drops vswhere.exe in
%ProgramFiles(x86)%\Microsoft Visual Studio\Installer and nothing adds
that directory to the PATH, so build scripts that locate VS with it -
and VsDevCmd.bat itself - print "'vswhere.exe' is not recognized" on
every run. New VsWhere step in the non-elevated half; it is skipped
with a warning when no VS installer is present.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DhsGf8BtmQE1PtS1CAKns4