When `IdleCount` is 0, machines are always created on-demand before
each job (if there is no available machine in Idle state). However,
when `IdleCount` were set to 0 and `limit` were set to a positive
value, the Runner would ignore the `limit` and attempt to allocate a
machine each time.

This change fixes that problem by updating `idleLimitStrategy` with a
new function, `canCreateOnDemand`, to complement `canCreateIdle`.  The
latter returns true if there is capacity to create idle machines for
pre-scaling, while the former returns true if there is capacity for
on-demand jobs.

We need to make this distinction to ensure that setting `idleCount` to
0 does not create an infinite number of idle machines in pre-scaling.

When a build attempts to `Acquire` a machine, we now call
`canCreateOnDemand` to check whether there is capacity to create a
machine. We need to do this check before the runner proceeds to the
`Prepare` stage where `Use` wou...