automatic build & test on pull requests

I recently added a GitHub workflow named =E2=80=9CBuild and test=E2=80=9D =
in the https://github.com/Clozure/ccl repository.

This is supposed to run automatically on pull requests.  I=E2=80=99m =
starting out with Linux only, since it=E2=80=99s the fastest and =
cheapest GitHub runner platform.

I=E2=80=99m not an expert on GitHub Actions, so I=E2=80=99d be happy to =
learn about doing things better, or to get help from someone who is good =
at this stuff.

The workflow file is =
https://github.com/Clozure/ccl/blob/master/.github/workflows/build-linux.y=
ml