Update repos/builtin (py-msgpack cxx --> c)

[climbfuji]

Description

Update submodule pointer for spack-packages (repos/builtin) for the changes in JCSDA/spack-packages#90: py-msgpack depends on c, not cxx.

Dependencies

• Change dependency from 'cxx' to 'c' for py_msgpack spack-packages#90

Issues addressed

spack/spack-packages#6586

Applications affected

Building unified environment on certain platforms.

Systems affected

Discovered on Narwhal and Nautilus.

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
  • Tested manually on Narwhal and Nautilus

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