There's a merged pull request on the void-packages repository

[RFC] xbps-src: default XBPS_MAKEJOBS to `nproc`
https://github.com/void-linux/void-packages/pull/40331

Description:
previously this defaulted to 1. In some rare cases, nproc is not desirable, but that can be easily overriden in the standard ways, etc/conf or -jN.

For compatibility with other distros, it defaults to 1 when nproc is not available.

#### Testing the changes
- I tested the changes in this PR: **YES**