Rebuild benchmarks on `-t` flag

[lekcyjna123]

To generate traces from cocotb benchmarks we need to pass -t flag, but additionally we have to manually clean the test/regression/cocotb/build directory, so that verilator files will be generated once more time. Fix that, so no manual intervention will be needed.

Implementation idea: In build directory store a file which contains last used flags to build verilator files, if flags have been changed, then will remove files from build directory.