libct/cg: retry opening a cgroup file #7052

Job	Run time
cross-i386	1m 34s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 34s
test (ubuntu-20.04, 1.22.x, rootless)	7m 25s
test (ubuntu-20.04, 1.22.x, -race)	7m 57s
test (ubuntu-20.04, 1.22.x)	7m 18s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 54s
test (ubuntu-20.04, 1.23.x, rootless)	7m 27s
test (ubuntu-20.04, 1.23.x, -race)	8m 8s
test (ubuntu-20.04, 1.23.x)	7m 33s
test (ubuntu-20.04, 1.23.x, criu-dev)	8m 3s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 49s
test (ubuntu-24.04, 1.22.x, rootless)	9m 22s
test (ubuntu-24.04, 1.22.x, -race)	8m 5s
test (ubuntu-24.04, 1.22.x)	7m 17s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 6s
test (ubuntu-24.04, 1.23.x, rootless)	9m 20s
test (ubuntu-24.04, 1.23.x, -race)	8m 15s
test (ubuntu-24.04, 1.23.x)	7m 32s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 2s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 48s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 24s
all-done	1s
	2h 48m 54s

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