In a File node, I have Geometry File set to
$HIP/../EgyptKit/Door_D.fbx

This works in Houdini, it opens the file fine. But then in Unity, I get the error:
Unable to read file "C:/Users/[user]/Houdini-Unity/../EgyptKit/Door_D.fbx"

It is as if $HIP points to the directory of my FruitMarket.hiplc file in Houdini, but then $HIP points to the root of the Unity project.

My Unity structure is like this:
Assets/ HoudiniAssets/ FruitMarket.hda FruitMarket.hiplc EgyptKit/ Door_D.fbx
Am I using the right environmental variable? Is there a different way to do this?

I ended up setting my project/$JOB in Houdini to the Unity project root to avoid any conflicts

In Houdini:
File -> Set Project -> (Choose the Unity project root)