Enable multiple cores in haskell GHA

[carbolymer]

Changelog

- description: |
    Enable multiple cores in haskell GHA
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Changes from cardano-api PR copied to cardano-cli:

• Enable multiple cores in haskell GHA cardano-api#629

Differences in build times: left before, right after this change:

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
• Self-reviewed the diff

[palas]

Looks good, and the CI passes, which is the important thing

[palas]

Why is the number in the cabal flag not specified but it is in the --ghc-option? I don't know how many cpus in the GHA runner, but I imagine it is a reasonable setting anyway

[carbolymer]

Github runners have 4 cores. I have no idea why it was not specified for cabal.
I vaguely remember we had issues when building too many packages concurrently, but I can't reproduce it anymore and -j4 was already maximum parallelism in GHA so it's kind of redundant.