ci: add CI via GitHub Actions

ci: add CI via GitHub Actions

- Add GitHub Actions CI on push and pull requests for:
  - Ubuntu 20.04 x86_64 gcc
  - Ubuntu 20.04 x86_64 clang
  - Ubuntu 20.04 i686 gcc
  - macOS 10.15
- Set the same dpkg flags as found in a default Ubuntu docker container.
  This has the following benefits:
  - Reduction in the amount of fsyncing dpkg/apt does
  - Installation/configuring of documentation is skipped
- On macOS Speed up homewbrew by not updating on install

v2:
- Use macOS 10.15 rather than 11.0 because 11.0 is only private preview
  (https://docs.github.com/en/actions/reference/specifications-for-github-hosted-runners
  ).
- Workaround "Could not perform immediate configuration on
  'libgcc-s1:i386'." on the Ubuntu 20.04 i686 configuration (see
  https://bugs.launchpad.net/ubuntu-cdimage/+bug/1871268/comments/170
  for details of the underlying issue).
- Install i386 development zlib.

Thanks to Lukasz Dorau for pointing the above out!

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>