Trouble building // Add github actions

[fire]

I would suggest adding a github actions for providing steps for at least one build variation to work.

[expenses]

To build babble I've ran this command with a binary download of LLVM:
cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=clang+llvm-17.0.6-x86_64-linux-gnu-ubuntu-22.04/ -DCMAKE_INSTALL_PREFIX=install && cmake --build build && cmake --install build
Then compiled bbl-usd with this:
cmake -B build -G Ninja -DCMAKE_PREFIX_PATH='../../babble/install' -DBUILD_EXAMPLE=on && cmake --build build

Note that I'm using Arch Linux which comes with an official USD package, so I don't need to set a path to that via CMAKE_PREFIX_PATH.