Specify max concurrency from command line

[NoxHarmonium]

Is your feature request related to a problem? Please describe.

I have a large project with heavy weight build processes.

At one point of our build, eight builds are running at once, and our build server falls over due to lack of memory.

Describe the solution you'd like

For a basic solution, specifying the maximum concurrency of a build from the command line would suffice.

However, it might be even better to be able to specify it in config as well.

Describe alternatives you've considered

Creating a fake dependency graph to force the builds to run in specific groups

Additional context

I have been working on a solution and should be able to open a PR to solve this issue shortly.

[ojkelly]

🎉 merged thanks!