make `-l` overridable

[NobbZ]

Describe the bug

The default usage of -l set to cores makes building on my system slow, as it usually has a load of 2 or 3, so make will not start more than one or two builds, despite the fact that there are more than 2 cores idle, as the load probably is due to IO wait.

Therefore it would be nice to be able to deactivate -l without individually overriding individual derivations or disabling its usage completely in builds.

[veprbl]

Have you seen https://nixos.org/manual/nix/stable/#chap-tuning-cores-and-jobs ?

[NobbZ]

I'm aware of max-jobs, though that doesn't really help. In general all nix related builds are basically serialized due to the usage of -l. Just throwing a bigger max-jobs won't help with that, as regardless of number of jobs that nix tries to run in parallel, make will now do more, just because the system load is above the threshold that is dictated by cores.

[avnik]

Upvote. It will be valuable for systems with lot of cores (20+) but slow spinning disks.

[veprbl]

See also #143820

[Artturin]

-l's were removed in #192447