isolate wheel-building in a shell script, run shellcheck

[jameslamb]

Contributes to #115

In #126, I'm working on moving the project's packaging metadata to pyproject.toml. To make that smaller and easier to review, this proposes some of the smaller auxiliary changes from there:

• putting all the wheel-building logic in a shell script
  • as is done across most of RAPIDS, e.g. https://github.com/rapidsai/cuml/blob/branch-24.08/ci/build_wheel.sh
• adding packaging-relevant file extensions to .gitignore
• running shellcheck in CI, to catch errors in shell scripts