fix: Fix builds in latest GitHub environments

[joeyparrish]

This fixes several (but not quite all) issues with building in GitHub Actions environments:

• macOS: Unlink additional conflicting libraries now installed by default through homebrew
• Windows: Install msys tools that used to be installed by default, but now need msys versions to be explicitly installed
• Windows: Drop gcc from update list to avoid hanging while installing packages (not sure why this works)
• Linux: Fix location of libaom pkg-config file
• All: Upload artifacts before checking them, so that the binaries can be debugged if they fail a later check

Remaining issues:

• FFmpeg build can't find libaom on Windows... but I plan to remove aom shortly.

[joeyparrish]

There's still one issue remaining, but I believe later PRs will address it.