Use nproc instead of /proc/cpuinfo to find number of processors for concurrency

[Haroon-Khel]

See adoptium/infrastructure#3360 (comment)

For docker containers, grep -c ^processor /proc/cpuinfo will return the number of processors of the host, not the container. Whereas nproc will return the number of processors of the container (given the container is run using the --cpuset-cpus="0-n" option.

nproc works on linux hosts so should not affect non docker nodes and it comes pre installed

[Haroon-Khel]

More importantly, jdk_time_2 was failling on some of our arm32 docker static containers because the test scripts read the cpu number of the host and so set -concurrency:81, https://ci.adoptium.net/job/Grinder/8700/console, causing the tests to overwhelm the container and timeout. nproc finds the correct number of cpus for the container, allowing the test to pass on our arm32 docker nodes https://ci.adoptium.net/job/Grinder/8710/console