TestPodSkipDevicesUpdate: open cpuset.cpus: ENOENT

[kolyshkin]

Saw this failure on Cirrus CI, running under centos-stream-8 (from https://github.com/opencontainers/runc/pull/3350/checks?check_run_id=4959472971)

=== RUN   TestPodSkipDevicesUpdate
    systemd_test.go:104: open /sys/fs/cgroup/cpuset/system.slice/system-runc_test_podTestPodSkipDevicesUpdate.slice/cpuset.cpus: no such file or directory
--- FAIL: TestPodSkipDevicesUpdate (0.07s)

I have not seen it before.

This must be some kernel race -- the code in question creates the directory and then reads its cpuset.cpus.

I am not sure what to do here, other than to retry on ENOENT.