I've never heard of a built-in way to execute MsBuild from webdeploy. I don't even think it's necessarily a valid idea to have such a built-in way as target machine can have several versions of msbuild co-existing (v2.0\3.5\4.5\etc) - how do you tell msDeploy which to use?
I suggest, that you use runCommand
and refer to msBuild using system variables: %WinDir%\Microsoft.Net\Framework\v4.0.30319\
Keeping in mind that framework versions move rather slowly you're safe (as you generally know, which version of the framework you target)
UPD: if you want a better way to find msbuild location - check out this answer (read from registry)