[ci] Limit to two workers

By default we run the prove task based on the number of available CPU cores.
The nprocs test will return the number of available CPU cores in the docker
executor VM, however the medium resource class only has access to
3 CPUs.

So the prove task will always spawn more workers than the number of available
CPU cores, which then causes the CPU to max out during the test and cause
random errors.

.circleci/config.yml

@@ -230,6 +230,7 @@ jobs:
             JUNIT_OUTPUT_FILE: tmp/junit/prove/report.xml
             TEST_NGINX_ERROR_LOG: tmp/junit/prove/error.log
             HARNESS: TAP::Harness::JUnit
+            NPROC: 2
       - report-codecov:
           suite: prove
           flags: -f 'luacov.report.*.out'