VS: Add an environment variable for the Windows 10 kits directory

Define a `CMAKE_WINDOWS_KITS_10_DIR` environment variable to allow
users to tell CMake about a custom Windows 10 SDK directory.  We
choose to make this an environment variable rather than a CMake
variable or cache entry because:

* Using a custom directory also requires custom external MSBuild
  configuration.  Therefore users are already configuring a
  custom environment.

* The custom directory must be set consistently in all parts of
  a build including nested projects.  An environment variable
  avoids requiring users to thread the setting into nested builds.

Fixes: #16743

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