Don't follow symlinks in append_dir_all() if follow_symlinks(false)

[nabijaczleweli]

Before (append_dir_all("file")):

called `Result::unwrap()` on an `Err` value: Os { code: 20, kind: NotADirectory, message: "Not a directory" }
called `Result::unwrap()` on an `Err` value: Os { code: 267, kind: NotADirectory, message: "The directory name is invalid." }

after (append_dir_all("file") or append_dir_all("symlink"):

called `Result::unwrap()` on an `Err` value: Custom { kind: NotADirectory, error: "append_dir_all() argument not a directory" }

Repro with

fn main() {
    let mut tar = tar::Builder::new(std::io::stdout().lock());
    tar.follow_symlinks(false);
    tar.append_dir_all("", std::env::args_os().nth(1).unwrap()) .unwrap();
    tar.finish().unwrap();
}

before:

$ ls -lnd src symlink Cargo.toml
-rw-r--r-- 1 1000 100 118 10-08 18:31 Cargo.toml
drwxr-xr-x 2 1000 100  60 10-08 17:57 src
lrwxrwxrwx 1 1000 100   3 10-08 18:30 symlink -> src
$ cargo run -- Cargo.toml | tar -tv
called `Result::unwrap()` on an `Err` value: Os { code: 20, kind: NotADirectory, message: "Not a directory" }
$ cargo run -- src | tar -tv
-rw-r--r-- 1000/100        211 2025-10-08 18:29 main.rs
$ cargo run -- symlink | tar -tv
-rw-r--r-- 1000/100        211 2025-10-08 18:29 main.rs

after:

$ cargo run -- Cargo.toml | tar -tv
called `Result::unwrap()` on an `Err` value: Custom { kind: NotADirectory, error: "append_dir_all() argument not a directory" }
$ cargo run -- src | tar -tv
-rw-r--r-- 1000/100        211 2025-10-08 18:29 main.rs
$ cargo run -- symlink | tar -tv
called `Result::unwrap()` on an `Err` value: Custom { kind: NotADirectory, error: "append_dir_all() argument not a directory" }

This also achieves parity with tar(1) which follow_symlinks() touts