[BUG] Flavour scripts don't work on non-persistent pots

[grembo]

Describe the bug
When setting the persistent flag to OFF in a flavour (set-attribute -A persistent -V OFF), running the flavour script fails.

To Reproduce
Create a flavour that contains a file that contains set-attribute -A persistent -V OFF and has a trivial flavour script flavour.sh. Try to create a pot and see it fail.

Expected behavior
Creating the pot should work.

Additional context
This used to work, probably by accident, since we had various sleeps inside pots that would keep it alive long enough to launch a command.

Since we no longer have background sleeps to keep the pot alive, it shuts down. The work to properly lock start/stop might also contribute to this.

A clean solution could be to override flv_set_cmd temporarily and run the non-persistent pot once - alternatively, start could grow a special parameter we can use for this case.

The workaround is to not create non-persistent pots (in case they are non-persistent for use in nomad, pot prepare overrides the persistent flag anyway).