Update Spare Instance parameter to consider idle executors

[Graham42]

If we're using more than 1 executor per agent and the scheduling strategy spreads builds across the existing agents, we could end up scaling up many more agents than we need.

[res0nance]

Could you add a test for this?

[Graham42]

I don't see any existing tests for this area of the code (unless I've missed something) and I'm not invested enough in this change right now to write a new test suite from scratch. If someone else wants to take on tests for this feel free.