Vastly overhaul the CI workflow for build checks (#852)

This workflow compiles TFQ and runs test cases to verify things work
after file changes are made by a PR or other event. It is a complete
rewrite of the previous TFQ build & test checks CI workflow, and
incorporates many new features and efficiency gains:

- Serious use of caching of the Python installation and environment, as
well as the Bazel environment, so that repeated runs don't have to
reinstall everything every time.

- File-aware execution: the workflow tests whether source files and
build configuration files have been modified in a given event, and stops
early if the event didn't affect such files.

- Compatibility with GitHub [merge
queues](https://github.blog/news-insights/product-news/github-merge-queue-is-generally-available/).

- Concurrency detection: if a new event such as a push to a PR branch
comes in while a workflow is already executing, that workflow is
cancelled and a new one restarted. (The existing workflow keeps the
existing one running and starts a second one, which is almost never
useful.)

- Addition of a manual run interface, allowing the workflow to be
invoked manually from the GitHub GUI with different values for program
versions – useful for debugging, testing, exploring behaviors with
different versions of linters, etc.

Author: mhucka