In the code coverage Buildkite job, do not use multiple threads (since we instead use multiple worker processes)

Author: DilumAluthge

.buildkite/pipelines/scheduled/coverage/coverage_linux64.yml

@@ -29,6 +29,7 @@ steps:
       git config --global init.defaultBranch master
 
       echo "--- Run Julia tests in parallel with code coverage enabled"
+      export JULIA_NUM_THREADS=1
       export JULIA_WORKER_TIMEOUT=1200 # 1200 seconds = 20 minutes
       ./julia -e 'import Distributed; @info "" Distributed.worker_timeout()'
       ./julia --code-coverage=all --sysimage-native-code=no .buildkite/pipelines/scheduled/coverage/run_tests_parallel.jl