build: run wix tool chain out of process

Building MSIs for different arch's can sometimes confuse MSBuild and
Wix, isntead run the toolchain externally so we don't have to worry
about which arch cmd.exe is running as.

tools/msvs/msi/nodemsi.wixproj

@@ -58,7 +58,7 @@
   </ItemGroup>
   <Import Project="$(WixTargetsPath)" />
   <Target Name="BeforeBuild">
-    <HeatDirectory ToolPath="$(WixToolPath)" Directory="..\..\..\deps\npm" PreprocessorVariable="var.NpmSourceDir" DirectoryRefId="NodeModulesFolder" ComponentGroupName="NpmSourceFiles" GenerateGuidsNow="true" SuppressFragments="false" OutputFile="..\..\..\npm.wxs">
+    <HeatDirectory ToolPath="$(WixToolPath)" Directory="..\..\..\deps\npm" PreprocessorVariable="var.NpmSourceDir" DirectoryRefId="NodeModulesFolder" ComponentGroupName="NpmSourceFiles" GenerateGuidsNow="true" SuppressFragments="false" OutputFile="..\..\..\npm.wxs" RunAsSeparateProcess="true">
     </HeatDirectory>
   </Target>
   <PropertyGroup>