libstore/globals.cc: Automatically set cores based on cgroup CPU limit

By default, Nix sets the "cores" setting to the number of CPUs which are
physically present on the machine. If cgroups are used to limit the CPU
and memory consumption of a large Nix build, the OOM killer may be
invoked.

For example, consider a GitLab CI pipeline which builds a large software
package. The GitLab runner spawns a container whose CPU is limited to 4
cores and whose memory is limited to 16 GiB. If the underlying machine
has 64 cores, Nix will invoke the build with -j64. In many cases, that
level of parallelism will invoke the OOM killer and the build will
completely fail.

This change sets the default value of "cores" to be
ceil(cpu_quota / cpu_period), with a fallback to
std::thread::hardware_concurrency() if cgroups v2 is not detected.

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