fix(fuzz): validate --fuzzcache-path against fuzzcachepath, not binpath

[MozirDmitriy]

This fix is necessary because the previous check validated the wrong variable, causing the script to erroneously fail when a valid fuzz cache path was provided (or pass when binpath existed but the fuzz cache path did not).

Arguments:

• The option explicitly sets fuzzcachepath and later is passed to -test.fuzzcachedir, so the check must target that variable.
• Docker usage creates and passes /fuzzcache to --fuzzcache-path, confirming the intended semantics.
• Other flags (--binary-path, --nitro-path) correctly validate their own variables; this brings --fuzzcache-path in line with that pattern.