use relative path to bash/python/ps
use relative path to bash/python/ps

The indent is to increase portability. NixOS/FreeBSD don't put bash in /bin/bash
and python in /usr/bin/python. macOS does but the bash version is very old -
many people probably prefer a different version i.e. homebrew's variant.

Part of pull request #523

hugsy/gef,hugsy/gef,hugsy/gef