Refactor: cleanup XDGRuntime

[apostasie]

From reading code, it seems to me like:

1. XDGRuntimeDir() should check the value of ROOTLESSKIT_PARENT_EUID (like ParentEUID does)
2. getXDGRuntimeDir() in bypass4netns is essentially duplicating XDGRuntimeDir()
3. getRuntimeVariableDataDir and CNIRuntimeDir should not ignore errors, and they seem to have faulty logic (relying on Geteuid regardless of conditions), or calling rootlessutil.ParentEUID which will redo stuff done already in XDGRuntimeDir()

PR addresses these.

[apostasie]

Was wondering if we should even incorporate into XDGRuntimeDir the extra logic

			if rootlessutil.IsRootlessChild() {
				return "", err
			}
			run = fmt.Sprintf("/run/user/%d", os.Geteuid())

but (for now) elected for pure refactoring with no change of behavior.

[apostasie]

Not clear why the failures (some of it is definitely the usual stargz / layer not found), but the rest is weird (eg: platform timeout).

If we can poke it in case it is a fluke...

[apostasie]

CI failure seems to be a timeout.

[apostasie]

Failures are a bunch of timeouts. Looks like the import cache manifest from github cache are extremely slow during the initial build phase:

• normal import ~ 1 second : https://github.com/containerd/nerdctl/actions/runs/13463005401/job/37622717754?pr=3850#step:4:90
• abnormal imports ~ 5 minutes+: https://github.com/containerd/nerdctl/actions/runs/13463005401/job/37622717494?pr=3850#step:4:72

Wether we should increase the overall timeout over 30 minutes or not is something we can talk about - at least these type of events would not fail the builds - but the underlying issue seem to be github action cache.

[apostasie]

Rebasing to force CI re-run.

[AkihiroSuda]

Warn should be still printed here?

[apostasie]

I don't think it is necessary, since err will be bubbled up and printed out as an error anyhow.

[apostasie]

eg: if we leave the warn, we will just print the error twice (once as a warn, and then as an error)

[AkihiroSuda]

Thanks

[apostasie]

CI failure looks like #3556

I am not sure why we are suddenly hitting that one a lot more.

Did we recently change something wrt cni?