tail: fix stdin redirect (#3842)

This fixes a bug where calling `tail - < file.txt` would result
in invoking `unbounded_tail()`.
However, it is a stdin redirect to a seekable regular file and
therefore `bounded_tail` should be invoked as if `tail file.txt` had
been called.

tests/by-util/test_tail.rs

@@ -76,6 +76,7 @@ fn test_stdin_redirect_file() {
         .set_stdin(std::fs::File::open(at.plus("f")).unwrap())
         .arg("-v")
         .run()
+        .no_stderr()
         .stdout_is("==> standard input <==\nfoo")
         .succeeded();