GitHub action to ensure BZFlag builds

[The-Noah]

This creates a GitHub Action that verifies BZFlag still compiles on push and in PRs.

[The-Noah]

Seems the script is broken, I'll take a look at fixing it.

[allejo]

Seems the script is broken, I'll take a look at fixing it.

Make sure to run apt-get update before you start installing packages.

[The-Noah]

Thanks! I've fixed it as seen here.

[blast007]

Our codeql-analysis.yml file already triggers a build on ubuntu-latest, though it currently does not build the client. This new action also installs SDL 1.2, which our 2.4 configure script won't use unless you explicitly tell it to use SDL1 via --with-SDL=1. So currently this new action also won't build the client. In any case, SDL2 is our default and preferred version.

Overall, my thought on this is that if it's only going to be building for Linux, we should just fix up the CodeQL action so that it runs a complete build. I don't know if there's any advantage to running a CodeQL analysis on multiple platforms. If there isn't, then a separate action like this one that builds on macOS and Windows (and Linux?) would be nice.