install-darwin: move nixbld gid to match first UID #103

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
meson_build (ubuntu-latest)	7m 25s
meson_build (macos-latest)	3m 46s
vm_tests	22m 23s
tests (ubuntu-latest)	30m 33s
tests (macos-latest)	42m 39s
installer	47m 4s
docker_push_image	0s
installer_test (ubuntu-latest)	40s
installer_test (macos-latest)	1m 14s
	2h 35m 45s

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