ln: rename error variant

[cakebaker]

While reviewing #7449 I noticed the following snippet:

if !target_dir.is_dir() {
    return Err(LnError::TargetIsDirectory(target_dir.to_owned()).into());
}

This PR fixes the snippet by renaming the error variant to TargetIsNotADirectory.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)