Way to build pybind11 in-tree without cmake fetch content

I have recently been trying to build python bindings on Roci and I'd like to build on the backend nodes, but the backend nodes don't have internet. This has produced this bizarre loop where I configure on the frontend and then build on the backend.

We should have a workaround for this, possibly just by having a pybind11 submodule for in-tree builds.