Run the build integ test in parallel (#39788)
Run the build integ test in parallel (#39788)

* Run the build integ test in parallel

Because the randomized runner lives in buildSrc, we run these tests with
the Gradle runner, and had no parallelism configured so far.

* Handle Windows and "auto" better

robin13/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,coding0011/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch