restrict serial-hdf5 builds to serial-hdf5

[minrk]

closes #139

ensures builds with nompi will also run with nompi, rather than allowing mpi builds, which are technically compatible.

This also specifies only the variant, fixing an inconsistency in the hdf5 version relative to conda-forge-pinning

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.