Not detecting custom paths

Description

When adding Project Specific Libraries and 3D models, the custom path isn't being used, instead it's hard coding the path to the files.

Steps to reproduce

1. Add a custom path to the project, in my case it's ${DDRAIGLIB} pointing to "C:\Dev\KiCAD\Ddraig68K\ddraig-libs"
2. Create a new project, and go to Manage Symbol Libraries and go to Project Specific Libraries and browse for existing library
3. When added to the project, the full path to the library is displayed, not the expected ${DDRAIGLIB}\ddraig.lib. This used to be detected correctly in KiCAD 5.

KiCad Version

Application: KiCad (64-bit)

Version: (6.0.0), release build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.78.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19044), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Dec 24 2021 19:17:02
	wxWidgets: 3.1.5 (wchar_t,wx containers)
	Boost: 1.76.0
	OCC: 7.5.0
	Curl: 7.78.0-DEV
	ngspice: 35
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON