One log file per invocation of k0sctl

Currently, k0sctl will produce one big log file. I wonder if it would be better to create a log file per invocation of k0sctl instead. I see the following benefits:

* Finding the right logs will be easier, because the log file grows over time and becomes a bit unwieldy.
* Cleaning up old logs is easier, while keeping more recent ones (based on the file name or file modification times).
* Multiple concurrent invocations of k0sctl won't interleave their log lines with each other.