Do not copy src for every single benchmark run

[cmnrd]

Currently, when performing a multirun with the benchmark runner, we copy the benchmark source files for each individual run into a new directory and compile from scratch. This is not only slow, but also duplicates build artifacts which can lead to considerable disk usage. I think it is important to copy the sources into a new (temporary) src tree so that we are sure that the original source tree is not modified by the runner and that any preexisting build artifacts don't interfere with the build. However, we should aim at doing this once and not for each individual run.