Prevent CMake targets from going stale.

[chiphogg]

Soon, we plan to support CMake (#215). We won't use globbing, because that's a can of worms. But several bazel targets do use globbing, so we need a way to make sure the CMake build doesn't go stale.

Maybe there's some GitHub action where the user will have to check a box to land the PR. And this blocking box can be triggered on:

• Any file being added in a directory, like units, that we glob.
• Any BUILD file being edited.

The latter would makes this issue more expansive than globs, so that it covers keeping CMake up to date generally.