Enable multiple cores in haskell GHA

[carbolymer]

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Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
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[smelc]

Suggested change

	run: cabal build all --enable-tests -j --ghc-option=-j4
	run: cabal build all --enable-tests -j --ghc-option=-j4 # Using 4 cores, because that's the number of processors of GitHub runners: https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories