use rapids-build-backend

[jameslamb]

Contributes to https://github.com/rapidsai/build-planning/issues/31 Contributes to https://github.com/rapidsai/build-planning/issues/69

Proposes:

• introducing rapids-build-backend as this project's build backend, to reduce the complexity of various CI/build scripts
• using pip install ./dist/*.whl instead of pip install --find-links ./dist in CI, to reduce the risk of the types of bugs described in https://github.com/rapidsai/build-planning/issues/69

Notes for reviewers

This project doesn't currently have a dependencies.yaml. rapids-build-backend does technically support that (https://github.com/rapidsai/rapids-build-backend/pull/33), so I chose not to introduce a dependencies.yaml file here just for that purpose.