Flaky test: TestRunWithSystemdTrueEnabled

> ```
> === Failed
> === FAIL: cmd/nerdctl/container TestRunWithSystemdTrueEnabled/waits_for_systemd_to_become_ready_and_lists_systemd_jobs (0.04s)
> container_run_systemd_linux_test.go:120:
>
> +============================================================================================================+
> | 🚀 | "TestRunWithSystemdTrueEnabled/waits_for_systemd_to_become_ready_and_lists_systemd_jobs": starti |
> | | ng test! |
> +============================================================================================================+
> | ⏳ | /tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/001/aab6ed55 |
> +============================================================================================================+
> | | |
> +============================================================================================================+
> | config | { |
> | | "Namespace": "nerdctl-test", |
> | | "PrivateMode": "" |
> | | } |
> +============================================================================================================+
> | labels | {} |
> +============================================================================================================+
>
> container_run_systemd_linux_test.go:120:
>
> +------------------------------------------------------------------------------------------------------------+
> | ➡️ | ⚙️ /usr/local/bin/nerdctl exec nerdctl-testrunwithsystemdtrueenabled sh -c -- tries=0 |
> | | |
> | | until systemctl is-system-running >/dev/null 2>&1; do |
> | | |
> | | >&2 printf "Waiting for systemd to come up...\n" |
> | | sleep 1s |
> | | tries=$(( tries + 1)) |
> | | [ $tries -lt 10 ] || { |
> | | >&2 printf "systemd failed to come up in a reasonable amount of time\n" |
> | | exit 1 |
> | | } |
> | | done |
> | | systemctl list-jobs |
> +------------------------------------------------------------------------------------------------------------+
> | | 🚫 command returned a non-zero exit code |
> +------------------------------------------------------------------------------------------------------------+
> | | ⚠️ 1 |
> +------------------------------------------------------------------------------------------------------------+
> | | 🟠 time="2026-02-15T06:17:31Z" level=fatal msg="OCI runtime exec failed: exec failed: unable to |
> | | start container process: error starting setns process: exec: already started" |
> +------------------------------------------------------------------------------------------------------------+
> | 🌱 | HOME=/root |
> | | PATH=/usr/local/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin |
> | | :/bin |
> | | DOCKER_CONFIG=/tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/003 |
> | | NERDCTL_TOML=/tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/003/n |
> | | erdctl.toml |
> +------------------------------------------------------------------------------------------------------------+
> | ⏰ | <1s (limit: 3m0s) |
> +------------------------------------------------------------------------------------------------------------+
> | 📁 | /tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/002 |
> +------------------------------------------------------------------------------------------------------------+
>
> container_run_systemd_linux_test.go:120:
>
> <<<<<<<<<<<<<<<<<<<<
> 🖊️ Command should succeed
> 👀 testing: `command returned a non-zero exit code`
> ❌ FAILED! is ``
> >>>>>>>>>>>>>>>>>>>>
> ```
>
> https://github.com/containerd/nerdctl/actions/runs/21416963179/job/63655402439?pr=4704
>
> Probably unrelated to this PR. Let's merge this
>

_Originally posted by @AkihiroSuda in https://github.com/containerd/nerdctl/issues/4704#issuecomment-3923759060_