Another round of experiment, with 10/18 cores on ARM/Intel Macs. (#1600)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index fc9b0a2..98e27bb 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1840,7 +1840,8 @@
     elif is_mac() and THIS_IS_TESTING:
         return "4"
     elif is_mac():
-        return "8"
+      # TODO(twerth): This is an experiment, remove.
+      return str(int(multiprocessing.cpu_count()/2))
     return "12"
 
 
@@ -2157,8 +2158,7 @@
     aggregated_flags = [
         "--flaky_test_attempts=3",
         "--build_tests_only",
-        # TODO(twerth): Remove after experiment.
-        # "--local_test_jobs=" + concurrent_test_jobs(platform),
+        "--local_test_jobs=" + concurrent_test_jobs(platform),
     ]
     # Don't enable remote caching if the user enabled remote execution / caching themselves
     # or flaky test monitoring is enabled, as remote caching makes tests look less flaky than
@@ -2190,8 +2190,7 @@
 def execute_bazel_coverage(bazel_version, bazel_binary, platform, flags, targets):
     aggregated_flags = [
         "--build_tests_only",
-        # TODO(twerth): Remove after experiment.
-        # "--local_test_jobs=" + concurrent_test_jobs(platform),
+        "--local_test_jobs=" + concurrent_test_jobs(platform),
     ]
     print_collapsed_group(":bazel: Computing flags for coverage step")
     aggregated_flags += compute_flags(