issues with pip install when building library from source for terra-jupyter-gatk img

[amygdala]

We're seeing issues with pip install for the terra-jupyter-gatk image when it's necessary to build the library from source.

It appears that PIP_TARGET is set when docker exec is run on the GCE instance: https://github.com/DataBiosphere/leonardo/search?q=PIP_TARGET
Whenever we run pip, it interprets the environment variable PIP_TARGET as the value for the --home flag. pypa/pip#8438 (comment)
BUT . . . whenever pip tries to install any packages from source, it passes flag --prefix and those flags cannot be used together.

By unsetting PIP_TARGET for an installation, we can work around the issue. e.g.:

!unset PIP_TARGET ; pip install -U docstring-parser==0.13

Nicole and I created a notebook to demo the issue: https://app.terra.bio/#workspaces/fc-product-demo/try-saturn/notebooks/launch/pip_install_issues.ipynb
The notebook html is attached in case it's useful.
pip_install_issues.html.zip

Other images may have this same issue, but we haven't tested yet.
Given the workaround, it's not urgent, but other customers are presumably hitting this as well. (I'll send an email with a bit more after the holidays).

/cc @deflaux

[deflaux]

I updated our tests to more closely reflect the pip environment we have in production. I then added a test to demonstrate this issue #286. You can see from the GitHub action test artifacts that the test fails, as expected. I've commented out the final assertion for now, and we can uncomment that assertion after this issue is fixed.

[Qi77Qi]

@amygdala @deflaux think this is resolved with https://github.com/DataBiosphere/leonardo/pull/2518....going to close the issue...but feel free to reopen if needed