stylix: make stylix-check's RAM usage configurable

[trueNAHO]

This is a PoC implementation of the following discussion:

We can try setting the number of threads for nix-fast-build first; it's currently set to the number of cores.

Ideally it would be

\min \left\{cores,\frac{memory}{X}\right\}

but we'd need a script to determine the memory size which works on both Linux and MacOS.

-- https://github.com/nix-community/stylix/pull/1069#issuecomment-2768860831

Before continuing working on this, I would like to get some feedback first. The TODO comments explain future plans.

Things done

• [X] Tested locally
• [ ] Tested in testbed
• [X] Commit message follows commit convention
• [ ] Fits style guide
• [ ] Respects license of any existing code used

Notify maintainers

@awwpotato, @danth