libct/cg: retry opening a cgroup file #6960

Job	Run time
cross-i386	1m 5s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 20s
test (ubuntu-20.04, 1.22.x, rootless)	6m 56s
test (ubuntu-20.04, 1.22.x, -race)	7m 51s
test (ubuntu-20.04, 1.22.x)	7m 23s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 21s
test (ubuntu-20.04, 1.23.x, rootless)	7m 2s
test (ubuntu-20.04, 1.23.x, -race)	7m 53s
test (ubuntu-20.04, 1.23.x)	7m 23s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 45s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 30s
test (ubuntu-24.04, 1.22.x, rootless)	9m 16s
test (ubuntu-24.04, 1.22.x, -race)	7m 31s
test (ubuntu-24.04, 1.22.x)	6m 34s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 7s
test (ubuntu-24.04, 1.23.x, rootless)	9m 16s
test (ubuntu-24.04, 1.23.x, -race)	7m 51s
test (ubuntu-24.04, 1.23.x)	7m 13s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 17s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 37s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 55s
all-done	1s
	2h 43m 7s

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