Environment variable to retrieve Houdini/Houdini Engine installation path in "HoudiniEngineEditor.GetHFSPath" method.

[KelSolaar]

Hi,

We are on a Windows pipeline where most of the software is available from a network share and the fact that the HoudiniEngineEditor.GetHFSPath method has no environment variable lookup beyond the registry and some hard-coded path is REALLY problematic. #78 touches quite rightly about this issue.

Cheers,

Thomas