nanobsd: Use the real path for NANO_PACKAGE_DIR

[jlduran]

Summary

As users may have the Pkg directory as a symbolic link to the NanoBSD
"package dump directory". In commit 9af130ae8c03, cust_pkgng() was
greatly improved, however as a side effect of using a nullfs mount, the
directories and files must not be symlinks.

Fix it by using NANO_PACKAGE_DIR realpath().

PR: 269884