Standard GitHub Actions runners now have four cores

[langston-barrett]

The current docs say that Linux runners have 4 cores for public repos, so this comment (and presumably the use of -j2) is outdated:

haskell-ci/src/HaskellCI/GitHub.hs

Lines 93 to 94 in f0fd898

	* We use -j2 for parallelism, as GitHub's virtual machines use 2 cores, per
	https://docs.github.com/en/free-pro-team@latest/actions/reference/specifications-for-github-hosted-runners#supported-runners-and-hardware-resources.