Persist Build Files Across Workflow Runs

[jrvollmer]

• Checkout the repository to a directory under the runner workspace that matches the PR's branch name (i.e. </path/to/workspace>/<branch-name>). Then, don't clean cmake_build/ between workflow runs. This way, compilation time can be reduced while keeping separate copies of the repo for each branch.
• On merge, delete the checked out repository from the runner's workspace
• On PR approval, clean cmake_build/ and rerun. While the PR is approved, clean cmake_build/ on every workflow run
  • The second part might be unnecessary since checks are required to pass and approvals have to be given on the most recent commit, which should change in the event that the checks failed, reran, and were successful the second time.