Define environment variable for overriding number of jobs

[giordano]

I'm happy to discuss alternative solutions, I'm not married to this one in particular, but the problem I want to solve is to be able to spawn more jobs than what default_njobs would do. Setting JULIA_CPU_THREADS only affects

ParallelTestRunner.jl/src/ParallelTestRunner.jl

Lines 361 to 363 in c79f4cc

	jobs = cpu_threads
	memory_jobs = Int64(free_memory) ÷ (2 * 2^30)
	return max(1, min(jobs, memory_jobs))

but that doesn't help anything, because the limitation is usually memory_jobs. Perhaps an alternative solution is to be able to control the expected memory usage more easily, which at the moment is impossible.

The only thing I like of using an environment variable to override the number of jobs is that that's easy to set in CI jobs, my main use case would be to have at least 2 parallel jobs on Apple Silicon macOS on GitHub-hosted runners, because at the moment we always get only one (3 CPUs and 7 GB of memory), which is a waste for most jobs.

[giordano]

Motivation for this PR was to be able to affect number of jobs without having to replace the julia-actions/julia-runtest workflow with a custom shell command call, but that may be possible with https://github.com/julia-actions/julia-runtest/blob/a3f37496e4b615530efe32573ddfa6c19712f5e7/README.md#pass-arguments-to-test-suite, I need to explore that.

[giordano]

Ok, setting the test_args option to julia-actions/julia-runtest works well, and I got some solid speedup in Enzyme tests: EnzymeAD/Enzyme.jl#2690 (review).

I'm going to close this PR, although the point about more flexible memory requirements for the tests still stands.