update hera spack-stack path

[RussTreadon-NOAA]

The spack-stack path is changing on Hera. This PR updates the path to the new location.

Resolves #159

[EdwardSafford-NOAA]

@RussTreadon-NOAA I confirmed the intel build (default) works correctly on hera with these changes. When I change build.sh to use gnu it doesn't work though. I've verified this is a pre-existing condition, not something introduced by these changes -- build in the develop branch currently fails with gnu. I didn't go very far into the weeds but at first glance I'm not sure spack-stack actually has support for gnu.

As for course of action I'm good with approving this PR, since the default condition builds correctly and I'm not sure anyone actually cares about gnu (one has to manually edit build.sh to get the gnu compiler). I can open a new issue to address the gnu build and handle that as a relatively low priority item. Thoughts?

[RussTreadon-NOAA]

@EdwardSafford-NOAA , thank you for testing this PR. I did not try the gnu build myself. It's good that you tested both feature/update_hera_path and develop. Like you, I am not aware of anyone using the gnu build.

I agree with your suggestion. A new issue can be opened to address the gnu issue. If we decide that we do not support the gnu build, we can close the issue.

[RussTreadon-NOAA]

Thank you, @EdwardSafford-NOAA. Merging now.

[EdwardSafford-NOAA]

I've added #163 to document the gnu build failure.

[RussTreadon-NOAA]

Great! Thank you @EdwardSafford-NOAA