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...