Update and fix Windows CI builds

- boost-spirit dependency needed to be installed
- Visual Studio wanted boost/filesystem.hpp included in more places
- a deprecated enum was removed from new versions of boost, so we use
  its replacement enum now
- vcpkg repository is now pinned to a commit where boost 1.84 libraries
  work smoothly, there should be no more surprises
- vcpkg dependencies now cache, so successive CI runs are dramatically
  faster
- for win-scons, we need to use 'call' in our batch script to source
  vcvarsall.bat
- I updated the includepath and libpath we pass to win-scons to match
  reality, but it still can't find any vcpkg libraries. This will need
  further sleuthing

.github/workflows/scripts/win/scons-build.bat

@@ -2,7 +2,7 @@
 setlocal enabledelayedexpansion
 
 for /f "usebackq tokens=*" %%i in (`vswhere -latest -products * -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -find **/Auxiliary/Build/vcvarsall.bat`) do (
-  %%i
+  call "%%i" x86_amd64
 )
 
 scons