[metricbeat][system/process] - process.cmdline.cache.enabled is not respected

When `cmdline.cache.enabled` is set to `true`, we should read process command and arguments once and re-use for later metrics.
For darwin and windows, we're fetching command args on every call. This should be fixed as per `process.cmdline.cache.enabled`'s value

Start in the Metricbeat system/process metricset and trace the Darwin and Windows paths that fetch command arguments. Compare their behavior with the process.cmdline.cache.enabled setting and run the relevant system/process tests; done means command and arguments are fetched once and reused when caching is enabled, while disabled behavior remains unchanged.

Written by the indexing model from the issue text.