Update pynacl install on spack-stack docker image build

[eap]

In the intel-impi docker image the external library pynacl was being installed for the native Python instead of the spack-stack Python. This change fixes the issue and has been verified in the built container 747101682576.dkr.ecr.us-east-2.amazonaws.com/jedi-intel-impi-dev:test

Testing

This was tested locally and was used in the updated intel testing image used in https://github.com/JCSDA-internal/fv3-jedi/pull/812

Applications / Systems affected

This change should primarily affect the jcsda-internal CI system.

Issue(s) addressed

https://github.com/JCSDA-internal/CI/issues/124

Checklist

• [x] This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• [x] These changes have been tested on the affected systems and applications.
• [x] All dependency PRs/issues have been resolved and this PR can be merged.