install-darwin: move nixbld gid to match first UID #104

Job	Run time
vm_tests	23m 0s
meson_build (ubuntu-latest)	7m 26s
Check Cachix and Docker secrets present for installer tests	1s
meson_build (macos-latest)	4m 18s
tests (ubuntu-latest)	30m 18s
tests (macos-latest)	38m 15s
installer	47m 46s
docker_push_image	0s
installer_test (ubuntu-latest)	34s
installer_test (macos-latest)	1m 6s
	2h 32m 44s

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