MSBuilder/MSVC: correct toolchain ver (#35424)

#35098 added the correct extraction of toolset version for the MSVC
compiler. This updates the associated method in MSBuilder to retrieve
the (now correct) property.

lib/spack/spack/build_systems/msbuild.py

@@ -77,7 +77,7 @@ def toolchain_version(self):
         Override this method to select a specific version of the toolchain or change
         selection heuristics.
         Default is whatever version of msvc has been selected by concretization"""
-        return self.compiler.msvc_version
+        return "v" + self.pkg.compiler.platform_toolset_ver
 
     @property
     def std_msbuild_args(self):