libct/cg: retry opening a cgroup file #7207

Job	Run time
cross-i386	58s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 13s
test (ubuntu-20.04, 1.22.x, rootless)	6m 56s
test (ubuntu-20.04, 1.22.x, -race)	7m 49s
test (ubuntu-20.04, 1.22.x)	6m 58s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 8s
test (ubuntu-20.04, 1.23.x, rootless)	6m 42s
test (ubuntu-20.04, 1.23.x, -race)	7m 36s
test (ubuntu-20.04, 1.23.x)	7m 0s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 38s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 23s
test (ubuntu-24.04, 1.22.x, rootless)	9m 5s
test (ubuntu-24.04, 1.22.x, -race)	7m 41s
test (ubuntu-24.04, 1.22.x)	6m 39s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 54s
test (ubuntu-24.04, 1.23.x, rootless)	10m 27s
test (ubuntu-24.04, 1.23.x, -race)	7m 49s
test (ubuntu-24.04, 1.23.x)	7m 24s
test (ubuntu-24.04, 1.23.x, criu-dev)	9m 12s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 36s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 4s
all-done	1s
	2h 43m 13s

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