Also report disk usage when running fuzzer

[mithro]

run_fuzzer.py helpfully reports CPU load and memory usage while running a fuzzer, see this code;

https://github.com/SymbiFlow/prjxray/blob/7325e99e0d2d120969abe242ed54d53e17e70bc8/fuzzers/run_fuzzer.py#L570-L578

This is super helpful in understanding when seemingly random things go wrong (like running out of memory). Given that disk space can be an issue, it would probably also be good to output current disk usage.

The only issue is that most people have many disks and you probably want to report only free space on the disk being used by the fuzzers?