Test

checks: disable `nixGarbageCollector` for multi-user #1178

Job	Run time
install-against-unstable	4m 28s
install-against-stable	15m 32s
install-flake-against-unstable	4m 1s
install-flake-against-stable	3m 6s
test-stable	4m 0s
test-unstable	10m 11s
	41m 18s

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