opts: set `-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}`

But only if `${DUNECI_PARALLEL}` is set.

Reference: https://gitlab.dune-project.org/infrastructure/issues0/issues/47
17 jobs for master in 65 minutes and 5 seconds (queued for 2 seconds)
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base:8-backports

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dune:2.3

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publish

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