We can use an extra matrix dimension to select which execution environment
we'll use for each GOOS/GOARCH pair, and thus avoid duplicating the
job definition for darwin just to set runs-on: macos-latest for it.

This is not really an intended use of a matrix dimension because it's
directly related to the existing "goos" one, rather than being an
independent third dimension, but it doesn't matter in practice because
we're using the "include" option to specify exact combinations, and thus
we're not relying on the built-in functionality to generate all possible
matrix combinations.