checks: disable nixGarbageCollector for multi-user

[zowoq]

Currently if you run activation as root you need to set nix.gc.user = "root"; or nix.gc.user = "";.

nix-darwin/modules/system/checks.nix

Lines 184 to 192 in 4b9b83d

	nixGarbageCollector = ''
	if test -O /nix/store; then
	echo "�[1;31merror: A single-user install can't run gc as root, aborting activation�[0m" >&2
	echo "Configure the garbage collector to run as the current user:" >&2
	echo >&2
	echo " nix.gc.user = \"$USER\";" >&2
	echo >&2
	exit 2
	fi

[zowoq]

Rebased to fix merge conflict.