GH Actions: Use pre-installed clang packages

Also fix the run by setting up the environment in non-deprecated way.

Always run with --debug to understand better what sort of stuff is happening in
the background. Also split out the bmake bootstrap stage (takes about 31s on
ubuntu, but 1m14 on macOS?)

Drops the dependency on coreutils (realpath, nproc) and thus (?) fixes macOS to
be just as fast (4 logical cores vs 2 physical cores before, go figure.)

Reviewed by: arichardson