Ability to perform a "clean build" on github PR workflow

[thomashampson]

Currently there is no way for a developer to specify whether to clean a machine (e.g. rm -rf build) on their PR build for the github workflow that runs linux built and test.

One workaround would be to replace a runner entirely, but that option is not available to most developers.

[glass-ships]

Don't github runners spin up a new container for each run? (i.e. aren't all builds clean?)