use rapids-build-backend

[jameslamb]

Contributes to rapidsai/build-planning#31
Contributes to rapidsai/build-planning#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 Remove uses of 'pip --find-links' in CI build-planning#69

Notes for reviewers

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