KISYS3DMOD and KICAD6_3DMODEL_DIR not used

Description

originally posted in #6952 (comment 480909296)

the KISYS3DMOD environmental variable isn't being used
and the path defaults to relative to the kicad binary's path
the KICAD6_3DMODEL_DIR environmental variable is accepted, but apparently not used

i'm setting both environmental variables to the same path
manually copying the value of KICAD6_3DMODEL_DIR to KISYS3DMOD in ``Configure Paths` results in working 3D models

Steps to reproduce

  1. have the 3D models only in a non-standard location
  2. set the KISYS3DMOD and KICAD6_3DMODEL_DIR environmental variables to that location
  3. open kicad
  4. see in Preferences -> Configure Paths... that:
    1. KICAD6_3DMODEL_DIR was taken from the environmental variable correctly
    2. KISYS3DMOD was not and defaulted to "relative to the kicad binary's path"
  5. open or create a project using a 3D model
  6. open the 3D viewer and see no 3D models (make sure to enable 3D models in the viewer settings)

KiCad Version

Application: KiCad

Version: 5.99.0-be542c05f2, release build

Libraries:
	wxWidgets 3.1.4
	libcurl/7.74.0 OpenSSL/1.1.1i zlib/1.2.11 libssh2/1.9.0 nghttp2/1.41.0

Platform: Linux 5.9.12 x86_64, 64 bit, Little endian, wxGTK, sway, wayland

Build Info:
	Date: Jan  1 1980 00:00:00
	wxWidgets: 3.1.4 (wchar_t,wx containers) GTK+ 3.24
	Boost: 1.69.0
	OCC: 7.5.0
	Curl: 7.74.0
	ngspice: 33
	Compiler: GCC 10.2.0 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON