Merge branch '1-use-nproc-build-jobs-as-makepkg-default' into 'master'

set default number of parallel make jobs to `$(nproc)`

Closes #1 
/spend 15m

See merge request !2
2 jobs for 2-use-arch-build-as-base-image in 1 minute and 18 seconds (queued for 4 seconds)