[CI] Fix failing mac builds

[tobil4sk]

For whatever reason, the Mac build started failing due to a SSL certificate error. While a temporary workaround could probably be found, a better option would probably be just to update to a newer version of MacOS, as homebrew no longer officially supports 10.14 either. This requires some issues to be solved as neko has compilation issues when building using xcode 12.

These were addressed in: #215

There are already PRs to fix most of these problems, which are:

• Fix MSG_NOSIGNAL warning on MacOS 11 #217
• libs/ui/ui.c -- fix build errors on Xcode 12 #218
• Fix Xcode 12 build error in libs/std/process.c #219

Then finally once those are merged, this PR does the remaining needed changes and also updates the CI. More details about the specific changes are available there:

• [CI] Fix mac builds #239

Theoretically if the CI succeeds after this then #215 should be resolved as well.

[Simn]

I have merged #217 through #219, please push to #239 again so it can run.

[tobil4sk]

Done.