Figure out a way to install `gcc-12` on github actions runners

Currently, the github actions script cannot build the program on POSIX *(Ubuntu & MacOS)* because neither have `gcc-12` in their stable APT repositories, and currently the program requires `gcc-12` or newer.  
I don't know if there is a way around that on a github actions runner, since we're running in a virtualized container... Figure that out!