do not show bootstrap command outputs on stderr when probing cpus
do not show bootstrap command outputs on stderr when probing cpus

on macOS, nproc doesnt exist and so bootstrap currently shows this
at build time:

```
./.duneboot.exe
/bin/sh: nproc: command not found
```

We now capture stderr in the bootstrap command invocation so that
the output is not sent onto the main console, making for a cleaner
build output on macOS.

Signed-off-by: Anil Madhavapeddy <3a02579c191f8558e3fedece867419cb15398b26@recoil.org>

janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder