CI: Add macOS test runners

[ryandesign]

This adds macOS test runners which were requested in a comment in #140.

I excluded the oldest nginx version, 0.8.55, from being run on the macOS test runners because it did not build; it said it required md5 support. The version of openssl installed may be so new that it no longer has md5 support, which is deprecated.

I also excluded gcc from being used on the macOS test runners because gcc is not included in macOS. (There is a gcc wrapper program but it just runs clang.) The Homebrew package manager is preinstalled in the macOS test runners along with several Homebrew packages including several versions of gcc, so if there is a strong desire to test with gcc on macOS it can be done, but it is not usual to use gcc on macOS. Clang is the preferred compiler.

I modified the Install Packages step to only run the apt commands on Linux. There is no apt on macOS. There is Homebrew (brew) but the packages we want are already installed. I figure the versions that are preinstalled are recent enough that I don't need to bother with trying to update them.

I removed CC=${{ matrix.compiler }} from the Test step's run section because it is redundant with its env section.

In the matrix section I moved dynamic below nginx (rather than above) because GitHub Actions seems to generate its job names based on the order in which the variables were defined, which in turn influences the order in which the builds are shown in the results. If this new ordering is not desired I can revert this part of the change. We can also specify the job name explicitly if we want it formatted differently.

I updated the mainline nginx version from 1.21.3 to 1.21.6 and the stable version from 1.20.1 to 1.20.2.

The macOS test runners will fail consistently until #145 is merged and all runners will fail intermittently until #146 is merged.

[ryandesign]

could you rebase

Here you go! Look at all those green check marks. ✅✅✅