Fix for PRODENV being set incorrectly in setup.sh

[slwatkins]

When running source setup.sh on some systems, the PRODENV variable is set instead to the folder where bash exists, e.g. /bin. This appears to be because the $0 is set to /bin/bash when sourcing the script (as opposed to running the script via bash setup.sh). I was experiencing this issue using a terminal on the NERSC Jupyter server.

See more, e.g., in this Stack Overflow post: https://stackoverflow.com/questions/21792176/0-doesnt-work-when-i-source-a-bash-script

I took the solution from one of the answers, and have tested it to confirm that PRODENV is set to location of the script when sourcing through the terminal on the Jupyter server, as well as ssh-ing into perlmutter directly.

[gipert]

But this does not work with zsh right? I was looking for a solution that support both bash and zsh but never had time to look into it

[slwatkins]

I ran it on my local mac with zsh and it worked fine, but using zsh on NERSC, I do get an error... Let me turn this PR into a draft, and I'll look into it a little more.