Detached benchmark launchers: export simulator/dumper home vars at run time, same env on both arms

A simv built with FSDB dump calls linked in (enabled by a runtime plusarg) still needs VCS_HOME/VERDI_HOME (and the tool bin on PATH) set when it runs, not just when it was compiled. Detached launch scripts (nohup/setsid under a guard wrapper) often start from a minimal non-login env where these are unset, so the dump arm fails or refuses at startup. Export them in the launcher itself, record the env to a file in the run root, and give the no-dump baseline arm the identical env so the pair differs only by the dump plusarg. Put a short smoke step (e.g. 10K cycles with dumping on) before the timed arms so a missing env stops the kit instead of burning the slot; and note that a baseline run with a different env than an older wave's baseline isn't strictly comparable across waves.