Fix platform name in Windows CI artifacts directory

Fix platform name in Windows CI artifacts directory

By renaming $Env:platform to $Env:project_platform. This avoids a
clash with the environment set up with vcvarsall.bat which changes
$Env:platform to the *architecture* which is 'x86' or 'x64' while we
use 'Win32' or 'x64' as the *project-level* platform.

From
https://docs.microsoft.com/en-us/visualstudio/ide/how-to-configure-projects-to-target-platforms?view=vs-2019:

"Note

The Win32 platform name is used for C++ projects, and it means
x86. Visual Studio considers both project-level platforms and
solution-level platforms, and the project platforms come from the
language-specific project systems. C++ projects use Win32 and x64, but
the solution platforms use x86 and x64. When you choose x86 as the
solution configuration, Visual Studio selects the Win32 platform for
C++ projects."

Fixes https://gitlab.com/graphviz/graphviz/-/issues/1803.