VS: Pass platform when invoking MSBuild

MSBuild expects a `/p:Platform=...` argument to tell it which platform
to build among those in the `.vcxproj` files.  We have not historically
had to do this because we generate only one platform.  However, when
a project uses `include_external_msproject` the included project file
may have other platforms.

Fixes: #18308
46 jobs for vs-msbuild-platform
Status Job ID Name Coverage
  External
passed cdash

passed cmake-terfin-windows-release-vs2017

passed ghostflow-branch-check-master

passed ghostflow-commit-check

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passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

00:33:16

passed ghostflow-stager

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