[wrappers] Don't re-source shellenv if already sourced

[mikeland73]

Summary

This fixes issue described by https://github.com/amithgeorge/devbox-nodejs-repro-20230406

It's not a perfect fix to all wrapper issues. Specifically:

USER=foo node -e "console.log(process.env.USER);"

will print the shellenv user instead of foo.

Will follow up with a fix for that.

cc: @amithgeorge

How was it tested?

Followed steps in https://github.com/amithgeorge/devbox-nodejs-repro-20230406

Added example that breaks in current release, passes in this PR

[loreto]

LGTM

Although tempted to move that example to testscripts/ instead ... it feels less like an example where we are teaching users how to use devbox in a particular use case, and more like a test-case designed to catch this edge case.

[mikeland73]

Although tempted to move that example to testscripts

@loreto yeah totally. I messed up