The original "afl" [1] has been abandoned and is not maintained since
2017. "afl++" [2] is its replacement. Since it's a fork, it has mostly
compatible command line, and more features.

[1]: http://lcamtuf.coredump.cx/afl/
[2]: https://github.com/AFLplusplus/AFLplusplus

The original afl is currently being phased out from distributions.
In particular, it's gone from whatever repo list GitHub Actions use,
breaking our build. Migrate to "afl++" instead.

Not all operating systems ship with "afl++" though. For example, it's
still not available in binary form on macOS via Homebrew. Therefore,
don't jump the gun and don't update the Makefile to use "afl++"-only
features. (However, it would be nice to use them if available.)

As noted in the comment, GitHub Actions runners are schizophrenic and
sometimes believe that they come from the future where the latest Ubuntu
image works correctly. Don't let them be fooled, use 18.04 for now.