realpath: don't require read permission on a directory just to confirm a trailing slash

[Chaganti-Reddy]

a trailing slash makes realpath assert the target is a directory, but that check was calling read_dir on it, which needs permission to list the directory's contents. GNU only needs a stat-level check there (search permission on the parent, not read on the target itself), so e.g. realpath /root/ works fine as a regular user even though they can't list /root.

swapped it for a metadata().is_dir() check, falling back to read_dir only when it's actually not a directory, so the existing 'Not a directory' error is unchanged.

Fixes #13151

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[cakebaker]

This looks odd. Why not return an error with ErrorKind::NotADirectory?

[Chaganti-Reddy]

if I just do ErrorKind::NotADirectory.into(), the message changes from GNU's "Not a directory" to rust's generic lowercase "not a directory" — the capitalized version we have now only exists because read_dir surfaces the real OS error text. want me to keep the exact GNU wording with Error::new(ErrorKind::NotADirectory, "Not a directory"), or are you fine with the plain rust message even though it drifts from GNU here?