[CI] Detect 64-bit Windows NSIS builds via pointer size, not generator platform (#7573)

CMAKE_GENERATOR_PLATFORM is Visual-Studio-only and stays empty under the
Ninja generator that Windows CI switched to in #7380, so TRICE_IS_64_BIT
was always 0 and every Windows artifact was mislabeled 32-bit. That fired
the installer's 'installing a 32-bit version on a 64-bit system' warning
and routed a genuinely 64-bit build into Program Files (x86) with the
32-bit registry view.

Use CMAKE_SIZEOF_VOID_P, which reflects the actual target architecture
for every generator (same test FindVCredistRuntime.cmake already uses).

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

CMakeLists.txt

@@ -327,7 +327,7 @@ if(UNIX)
   endif()
 elseif(WIN32)
   set(CPACK_GENERATOR NSIS ${CPACK_GENERATOR})
-  if("${CMAKE_GENERATOR_PLATFORM}" MATCHES "(x64)")
+  if(CMAKE_SIZEOF_VOID_P EQUAL 8)
     set(TRICE_IS_64_BIT 1)
   else()
     set(TRICE_IS_64_BIT 0)