Set default value for PARALLEL_LEVEL to nproc

[jjacobelli]

The PARALLEL_LEVEL variable is used in CI to set parallel level while building. Currently this variable is set on the GHA runners to nproc, but as this is a RAPIDS specific variable, this variable will soon be removed from the runners, so RAPIDS projects must explicitly set this variable to nproc. This PR is setting the default value of PARALLEL_LEVEL to nproc to match what is done by other RAPIDS projects