libct: fix user ns join order for rootful container #6892

Job	Run time
cross-i386	1m 0s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 7s
test (ubuntu-20.04, 1.22.x, rootless)	7m 19s
test (ubuntu-20.04, 1.22.x, -race)	7m 53s
test (ubuntu-20.04, 1.22.x)	6m 53s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 50s
test (ubuntu-20.04, 1.23.x, rootless)	7m 11s
test (ubuntu-20.04, 1.23.x, -race)	7m 59s
test (ubuntu-20.04, 1.23.x)	7m 36s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 47s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 43s
test (ubuntu-24.04, 1.22.x, rootless)	10m 31s
test (ubuntu-24.04, 1.22.x, -race)	7m 31s
test (ubuntu-24.04, 1.22.x)	6m 39s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 43s
test (ubuntu-24.04, 1.23.x, rootless)	9m 23s
test (ubuntu-24.04, 1.23.x, -race)	7m 53s
test (ubuntu-24.04, 1.23.x)	7m 10s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 39s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 30s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 52s
all-done	0s
	2h 45m 9s

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