nanobsd: Fix parallel make

The NANO_NCPU (number of CPUs) value gets ignored by the build script
when setting the NANO_PMAKE (parallel make) value.

Fix it by setting the NANO_PMAKE later in the process.

PR: 277269
Reviewed by: imp, emaste
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47476