Evaluate using `Popen(shell=False, ...)`

[smarr]

Should we avoid using the shell for running commands?

Think this is a major change at the core of ReBench.
So, we'd probably need to carefully test this.
See #248 (comment)

[smarr]

At the moment, we request explicitly the use of a shell here:
https://github.com/smarr/ReBench/blob/master/rebench/executor.py#L532

However, we run build commands without shell here:
https://github.com/smarr/ReBench/blob/master/rebench/executor.py#L397