This PR injects the 'NOMAD_CPU_CORES' environment variable into
tasks that have been allocated reserved cpu cores. The value uses
normal cpuset notation, as found in cpuset.cpu cgroup interface files.
Note this value is not necessiarly the same as the content of the actual
cpuset.cpus interface file, which will also include shared cpu cores when
using cgroups v2. This variable is a workaround for users who used to be
able to read the reserved cgroup cpuset file, but lose the information
about distinct reserved cores when using cgroups v2.
Side discussion in: https://github.com/hashicorp/nomad/issues/12374
8cfe123a3a