setup-nvidia: Make "current" symlink logic more robust

Users have reported that in some cases the nvidia.service fails because
/opt/nvidia/current is a directory and the symbolic link gets created inside
it. I have no idea how we get there, but to make the service robust in the face
of this kind of issue:

- remove the directory if it exists
- use `-T` with ln to ensure that symbolic link creation fails if `current` is a directory

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

sdk_container/src/third_party/coreos-overlay/x11-drivers/nvidia-drivers/files/bin/setup-nvidia

@@ -88,7 +88,11 @@ SYSEXT_LEVEL=1.0
 EOF
 
   pushd /opt/nvidia
-  ln -sfn "${NVIDIA_FLATCAR_VERSION_PAIR}" current
+  if [[ -d "${NVIDIA_CURRENT_INSTALLATION}" ]]
+  then
+    rm -rf "${NVIDIA_CURRENT_INSTALLATION}"
+  fi
+  ln -sfn -T "${NVIDIA_FLATCAR_VERSION_PAIR}" "${NVIDIA_CURRENT_INSTALLATION}"
   popd
 }