Creating symlinks on Windows requires administrator privileges or
Developer Mode. Replace the direct os.Symlink/os.Readlink calls with
platform-specific helpers: unix keeps the existing symlink behavior,
while Windows creates a directory junction using go-winio, which does
not require elevated privileges.

os.Readlink works for both symlinks and junctions, so resolveWorkLink
uses it on both platforms.

Fixes "A required privilege is not held by the client" error when
creating containerd tasks on Windows without elevated privileges.

Signed-off-by: Craig Gumbley <craiggumbley@gmail.com>