ci: dynamically match makejobs with cores

Previously jobs were running on a large multi-core server where 10 jobs
as default made sense (or may even have been on the low side).

Using hosted runners with fixed (and lower) numbers of vCPUs we should
adapt compilation to match the number of cpus we have dynamically.

This is cross-platform compatible with macos and linux only.

.github/workflows/ci.yml

@@ -20,7 +20,6 @@ concurrency:
 env:
   CI_FAILFAST_TEST_LEAVE_DANGLING: 1  # GHA does not care about dangling processes and setting this variable avoids killing the CI script itself on error
   CIRRUS_CACHE_HOST: http://127.0.0.1:12321/ # When using Cirrus Runners this host can be used by the docker `gha` build cache type.
-  MAKEJOBS: '-j10'
   REPO_USE_CIRRUS_RUNNERS: 'bitcoin/bitcoin' # Use cirrus runners and cache for this repo, instead of falling back to the slow GHA runners
 
 defaults:

ci/test/00_setup_env.sh

@@ -35,7 +35,7 @@ fi
 
 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
-export MAKEJOBS=${MAKEJOBS:--j4}
+export MAKEJOBS=${MAKEJOBS:--j$(if command -v nproc > /dev/null 2>&1; then nproc; else sysctl -n hw.logicalcpu; fi)}
 # Whether to prefer BusyBox over GNU utilities
 export USE_BUSY_BOX=${USE_BUSY_BOX:-false}