make build-arg passing stricter

[jameslamb]

Contributes to https://github.com/rapidsai/build-planning/issues/7.

• removes some defaults in ARG statements in the project's Dockerfiles
• replaces other defaults with placeholder values that make loud errors more likely

See https://github.com/rapidsai/ci-imgs/pull/102 for a description of why I'm proposing this.

In short, I think it's desirable for builds to fail with a big loud error if the expected build args aren't passed through.

e.g. this:

docker-buildx build \
    -f ./Dockerfile \
    -t rapidsai/miniforge-cuda:local \
    .

Should result in

ERROR: failed to solve: nvidia/cuda:notset-base-notset: docker.io/nvidia/cuda:notset-base-notset: not found