fix `NIX_PATH` for real #11090

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
vm_tests	15m 15s
meson_build (ubuntu-latest)	11m 8s
meson_build (macos-latest)	4m 9s
tests (ubuntu-latest)	33m 58s
tests (macos-latest)	25m 1s
installer	0s
docker_push_image	0s
installer_test	0s
	1h 29m 32s

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