Setup of DIRACOS does not restrict cpu cores

[maxfischer2781]

When the Pilot creates its DIRACOS environment, it directly calls the DIRACOS-Linux-machine.sh which eventually invokes mamba. Mamba assumes that it can use all cores of the machine (see mamba-org/mamba#2463), which isn't realistic for Pilot environments. This leads to excessive process creation, which can negatively affect the pilot, user or even entire compute resource.

As far as I can tell, the templates from which DIRACOS is generated do no provide a feasible way to limit this internally. The Pilot thus seems like the best place, seeing how it is aware of resource restrictions.
A solution would be to set MAMBA_EXTRACT_THREADS when installing DIRACOS, either to a conservative 1 or pp.maxNumberOfProcessors.

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For reference of scale, we caught this on a WLCG Tier 1 WN with 256 cores that got allocated mostly to one VO. Each of the single core pilots tried to use 256 child processes; each pilot quickly ground to a halt due to resource and fork bomb protection, which caused each new pilot to also immediately get stuck on nproc limits and similar safeguards.