Build Automation

A while back I implemented path variable overrides for my InstallScript MSI projects.
I added this to my isproj files:

<ItemGroup>
<InstallShieldPathVariableOverrides Include="$(MSBuildRoot)">
<PathVariable>PATH_TO__SOURCE_ROOT</PathVariable>
</InstallShieldPathVariableOverrides>
</ItemGroup>

I have been building from a bat file by passing the property MSBuildRoot="D:\cn10_2013q4" to MSBuild.exe and that has been working fine.

Now we are implementing Build Automation with VS 2013 and InstallShield 2013 SP1 SAB.
In my build definition for '5. Advanced - MSBuild Arguments' I specified /Property:MSBuildRoot=$(MSBuildThisFileDirectory)
I am getting this error:

C:\Program Files (x86)\MSBuild\InstallShield\2013\InstallShield.targets (77): PathVariable "PATH_TO__SOURCE_ROOT" path "$(MSBuildThisFileDirectory)" does not exist.

I have tried several properties similar to $(MSBuildThisFileDirectory) and get the same result.
How do I resolve?
Thanks.