ci: fix delete.bats for GHA

[kolyshkin]

TL;DR: this is a forward-port of https://github.com/opencontainers/runc/pull/3538/commits/f46c0dad655e33cf29d38da94b265d982a835f88 (part of #3538) to main branch, fixing a CI flake caused by a GHA CI env peculiarity.

A couple of test cases in delete.bats check that a particular cgroup exists (or doesn't exist) using find. This is now resulting in errors like these:

    find: ‘/sys/fs/cgroup/blkio/azsec’: Permission denied
    find: ‘/sys/fs/cgroup/blkio/azsec_clamav’: Permission denied
    find: ‘/sys/fs/cgroup/cpu,cpuacct/azsec’: Permission denied
    find: ‘/sys/fs/cgroup/cpu,cpuacct/azsec_clamav’: Permission denied
    find: ‘/sys/fs/cgroup/memory/azsec’: Permission denied
    find: ‘/sys/fs/cgroup/memory/azsec_clamav’: Permission denied

leading to test case failures.

Apparently, GHA runs something else on a test box, so we get this.

To fix, ignore non-zero exit code from find, and redirect its stderr to /dev/null.