Where does Powershell and PSake execute msbuild from?
PSake supports more MSBuild versions on one box. You can choose version and platform eg. 4.0x64 with framework directive. Path to MSBuild tools is not taken from $env:path but is computed. It uses registry to find proper path.
VS 2013 and MSBuild Tools are supported since PSake 4.3
In PS type [Environment]::GetEnvironmentVariable("Path")
Do you see both paths? In what sequence do they appear? What occurs in cmd vs ps when you switch the order?