Grepping `/proc/cpuinfo` does not yield the correct number of available
cpus when running in a docker container or setting the number of
available cpus with taskset.
```
$ taskset 1 grep -c processor /proc/cpuinfo
8
$ taskset 1 nproc
1
```
This will prevent using too many build jobs on environments where the
number of available cpus is reduced.
`nproc` is part of `coreutils`.
Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>