qa_eeschema fails to find ngspice

Description

in my nix package, qa_eeschema started to fail due to not finding the ngspice library, even though it is provided
(at buildtime, ngspice is detected: -- Found ngspice: /nix/store/nzd4i6xfxmwqqq2hwm5g9sb2sxdn0hcf-libngspice-37/include
at runtime, simulation seems to work)
this seems to have started somewhere between 2022-06-21 and 2022-09-18

Running 86 test cases...
/build/source/qa/unittests/eeschema/test_netlist_exporter_spice.cpp(111): fatal error: in "NetlistExporterSpice/Rectifier": critical check ngspice has failed
/build/source/qa/unittests/eeschema/test_netlist_exporter_spice.cpp(80): error: in "NetlistExporterSpice/Rectifier":
NGSPICE LOG
===========

09:26:11: Error: Missing ngspice shared library

this error occurs 14 times on a run of qa_eeschema, all in "NetlistExporterSpice"

i'm hoping someone knows if something about the library detection around this changed in the last few months

Steps to reproduce

1. build KiCad as described here, minus the LD_LIBRARY_PATH line: https://github.com/NixOS/nixpkgs/pull/191887

KiCad Version

Application: KiCad

Version: 6.99.0-e4865ce543, release build

Libraries:
	wxWidgets 3.1.5
	FreeType 2.12.1
	HarfBuzz 5.1.0
	FontConfig 2.14.0
	libcurl/7.84.0 OpenSSL/3.0.5 zlib/1.2.12 brotli/1.0.9 zstd/1.5.2 libidn2/2.3.2 libssh2/1.10.0 nghttp2/1.47.0

Platform: Linux 5.15.62 x86_64, 64 bit, Little endian, wxGTK, , wayland

Build Info:
	Date: Jan  1 1980 00:00:00
	wxWidgets: 3.1.5 (wchar_t,wx containers) GTK+ 3.24
	Boost: 1.79.0
	OCC: 7.6.2
	Curl: 7.84.0
	ngspice: 37
	Compiler: GCC 11.3.0 with C++ ABI 1016

Build settings:
	KICAD_USE_EGL=ON
	KICAD_SPICE=ON