Make output target display not have local targets hanging around so much

[peterebden]

This is split out from another PR I'm working on, to try to keep that one a bit more sane.

What I'm observing is that while I have local build actions running at a fair pace, with remote execution online but being underutilised, the display shows a lot more local actions than I'd expect. It isn't actually executing them, just showing their 'shadow' for a few seconds.

This happens because the display is using a common set of 'slots' for them which is shared between both. This changes it so each has its own set of 'available slots' that are not shared. The eventual result is that you see a few local actions at the top of the list, to a max of the number of local workers that are configured, with all the remote ones beneath them.

This is what was intended all along but it wasn't quite doing it; I don't think we have anywhere where we'd really observe it right now, you'd need something like remote execution enabled with a whole bunch of actions with local = True on them (we have no such actions internally, it's been a requirement that everything supports remote execution for a few years now).