neptune-dev-nvhpc updates for Blueback

[climbfuji]

Description

This PR adds a nvhpc@26.5 configuration on Blueback and further strips down the neptune-dev-nvhpc template so that it builds under the environment module system constraints on Blueback. Previously, I built neptune-dev-nvhpc on my laptop where it was possible building py-numpy with gcc; On Blueback, that's not an option, because of module conflicts between the primary compiler (nvhpc) and gcc. It is also necessary to suppress the generation of nvhpc compiler modules in Spack in the same way as we do it for the other compilers already.

Dependencies

None

Issues addressed

Follow up to #2094

Applications affected

None.

Systems affected

Blueback

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped (provide explanation if necessary)
• New tests added: List and describe any new tests added to GitHub actions
  • ...
• Additional testing: Add information on any additional tests conducted
  • Built NEPTUNE nvhpc environment on Blueback with nvhpc@26.5, ran smoke test

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the documentation (spack-stack wiki) will be made when this PR is merged

[climbfuji]

To avoid duplicate packages because of the mixed GCC-NVHPC environment. Since it's in packages_nvhpc.yaml, it won't affect any of the existing builds/compilers.