Headless mono build not detecting installed mono libs

[andrewdbanks]

Godot version:
master, commit b67955a

OS/device including version:
Kubuntu 18.04

Issue description:
I was following the build instructions for building Godot with mono, but when I run the output executable against my Hello World C# project, I get the following output in my terminal:

The assembly mscorlib.dll was not found or could not be loaded.
It should have been installed in the `/media/andrew/UbuntuStorage1/Godot-Mono/godot-master/bin/data_Godot/Mono/lib/mono/4.5/mscorlib.dll' directory.

Steps to reproduce:

1. Install mono using the repositories found at https://www.mono-project.com/download/stable/
2. Create a Godot project (using the Mono binary available for download) with a GDScript that calls a simple C# class. In the C# class, include a Console.write() call to display "Hello World".
3. Follow Godot build instructions at https://docs.godotengine.org/en/latest/development/compiling/compiling_with_mono.html#requirements
4. Run the output Godot executable from step 3 against the project from step 2.

Minimal reproduction project:
HiredGuns.zip

[aaronfranke]

@andrewdbanks I just tested this on Godot 3.2.3 and I can't reproduce the issue. If you are still experiencing this issue, please test if this issue still exists with Godot 3.2.3, the 3.2 branch, and/or the master branch.

Here is the minimal reproduction project updated to Godot 3.2.3: HiredGunsBut3.2.3.zip