This commit adds an extra (optional) step for the Windows
installation/set-up to include the containerd binaries in
the $env:Path so that later executions especially
for `ctr.exe` if needed, do not require to specify the full path.
It also further fixes the previous steps to be absolute and
also work with re-installations and upgrades.
Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>