Preserve LD_LIBRARY_PATH in start.sh

[mathbunnyru]

Describe your changes

Issue ticket if applicable

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes

[mathbunnyru]

I created this Dockerfile and built it with docker build -t test .:

FROM jupyter/docker-stacks-foundation

ENV LD_LIBRARY_PATH=/test/

Then I ran: docker run -it --rm -e NB_USER=some-user --user root test start.sh
LD_LIBRARY_PATH was lost.
I think it's a good idea to keep it - people might install software, that might need LD_LIBRARY_PATH changes.
If they change NB_USER, then their image will stop working properly.

This PR fixes this issue.

[mathbunnyru]

Credits to @benz0li in this PR: #1979

[benz0li]

Cross reference: iot-salzburg/gpu-jupyter#108 (comment) ff