Fix the load script to reinstall polaris when needed

[xylar]

It was looking for setup.py but this has been replaced by pyproject.toml in the root of the repo.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran ./configure_polaris_envs.py and then sourced the load script. As expected, I got:

$ source load_polaris_test2_chrysalis_intel_openmpi.sh 
Loading conda environment
Done.

Reinstalling polaris package in edit mode...
Done.

Loading Spack environment...
Done.

Without this fix, we were not seeing Reinstalling polaris package in edit mode....