support rapids-build-backend

[jameslamb]

Contributes to rapidsai/build-planning#31

• adds rapids-build-backend flags for pip builds
• updates rapids-python-uses-scikit-build-core to correctly detect scikit-build-core even when rapids-build-backend is the top-level build-backend

How I tested this

Rebuilt the container, then ran the following in the CUDA 12.2 pip container.

# clean previous build outputs
uninstall-all -j
clean-all -j

# work on latest branches of everything
rapids-checkout-same-branch --omit ucxx
git -C ./ucxx checkout branch-0.39
rapids-pull-repositories

# install all the other dependencies
rapids-make-pip-env --force

# build all the packages
build-all -j

# run the cudf unit tests
cd ./cudf
./ci/run_cudf_pytests.sh

# run the cuml unit tests
cd ./cuml
./ci/run_cuml_singlegpu_pytests.sh

Saw that all succeed.

[trxcllnt]

Looks like I messed up the history trying to rebase out the commits from main. Luckily this will go away when we squash!

edit: fixed the git history

[jameslamb]

/merge