Running linters/testsuite with hadrian with -j leads to overcontention

If you run the command

"hadrian/build" -j8 test --test-root-dirs="testsuite/tests/linters" lint:compiler lint:base

Then the testsuite, and both linters will be started with -j8 as an argument which leads to horrible overcontention.

Relevant shake issue is: https://github.com/ndmitchell/shake/issues/491

Blocking #20614