Merge dashpay#6489: ci: allow overriding MAKEJOBS

feb6a1a ci: allow overriding MAKEJOBS (UdjinM6)

Pull request description:

  ## Issue being fixed or feature implemented
  That's how these variable are ment to behave
  >echo "Fallback to default values in env (if not yet set)"

  https://github.com/dashpay/dash/blame/develop/ci/test/00_setup_env.sh#L26

  That's also how it's done in bitcoin https://github.com/bitcoin/bitcoin/blob/master/ci/test/00_setup_env.sh#L38.

  But we broke it in c52992a and I'm not sure why 🤷‍♂️

  ## What was done?

  ## How Has This Been Tested?

  ## Breaking Changes

  ## Checklist:
  - [ ] I have performed a self-review of my own code
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have added or updated relevant unit/integration/functional/e2e tests
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have assigned this pull request to a milestone _(for repository code-owners and collaborators only)_

ACKs for top commit:
  PastaPastaPasta:
    utACK feb6a1a
  kwvg:
    utACK feb6a1a

Tree-SHA512: 617e761cf5c52d8e4329bf8aa1680c72a7816ae3248ed415d2c3232e5aa0941f18e21f0d6d363848238f53730097aa22f51a6e1cd62bd0b31d3da853f9f79e28

ci/test/00_setup_env.sh

@@ -25,8 +25,7 @@ export JOB_NUMBER=${JOB_NUMBER:-1}
 
 echo "Fallback to default values in env (if not yet set)"
 # The number of parallel jobs to pass down to make and test_runner.py
-MAKEJOBS="-j$(nproc)"
-export MAKEJOBS
+export MAKEJOBS=${MAKEJOBS:--j$(nproc)}
 # A folder for the ci system to put temporary files (ccache, datadirs for tests, ...)
 # This folder only exists on the ci host.
 export BASE_SCRATCH_DIR=${BASE_SCRATCH_DIR:-$BASE_ROOT_DIR/ci/scratch}