[potentially breaking] update to use pybind11 submodule

[Yurlungur]

PR Summary

I am setting up CI for riot, which does not use spack, and I am finding that including pybind11 via git submodules is less painful than via pybind11. I think because we do everything via spack or via git submodules this should not be breaking but we should double check by making sure all the CI passes.

This does make pybind11 more in-line with how we treat other dependencies in singularity-eos so I think it's reasonable.

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI
• If ML was used, make sure to add a disclaimer at the top of a file indicating ML was used to assist in generating the file.
• If Agentic AI was used, have the AI generate a "proposed changes" markdown file and store it in the plan_histories folder, with a filename the same as the MR number.

If preparing for a new release, in addition please check the following:

• Update the version in cmake.
• Move the changes in the CHANGELOG.md file under a new header for the new release, and reset the categories.
• Maintainers: ensure spackages are up to date:
  • LANL-internal team, update XCAP spackages
  • Current maintainer of upstream spackages, submit MR to spack

[adamdempsey90]

The replacement of the FetchContent stuff from before with just add_subdirectory seems like the correct thing to do but the CI will be the arbiter

[Yurlungur]

Tests passing on re-git. Merging.