Can't use --parameters and --run_config at the same time with exaust.py

[HackerFoo]

I tried to get multiple samples running VPR with different parameters (just one set) from the defaults. It seemed that combining a run config with multiple build numbers, and a parameters file with the flags I wanted would do this, but instead this assertion fails, because args.toolchain is None: https://github.com/SymbiFlow/fpga-tool-perf/blob/master/exhaust.py#L246

Which it must be because of: https://github.com/SymbiFlow/fpga-tool-perf/blob/master/exhaust.py#L207

Furthermore, args.toolchain and args.project are used on test failure even if using --run_config.

[HackerFoo]

In my case, it would be sufficient just to add a --params_string flags, but for now I've just modified exhaust.py since it's a one-off experiment. If I get time, I'll add the flag.