Optimize GitHub Actions billable time

[memcorrupt]

NOTE: This issue probably may never be relevant since this repository uses the free tier of GitHub Actions.

Currently, the matrix build used here only takes ~10 seconds per build. Since GitHub actions rounds up billable time to the nearest minute per runner, we end up incurring significantly more billable time than actually used. This can be resolved by combining some of these jobs (e.g, all Lua 5.x builds for a certain OS can be grouped into one sequential job instead of queuing them concurrently).

Additionally, since Linux runners are cheaper than Windows runners, and Windows runners cheaper than macOS runners, billable time could be further optimized by cross-compiling on cheaper runners.

Also, better usage of all of GitHub actions cores could make each compilation run faster (usingmake -j option).