`brew test-bot` can occasionally spend a long time doing nothing when
testing dependents.[^1] This is because it takes a long time to work out
that there is nothing to do.

To fix this, let's adjust `brew determine-test-runners` to pass on only
the list of formulae for which there is work to be done so that
`brew test-bot` doesn't need to waste time working this out.

[^1]: For example, it spends about 15 minutes doing nothing at https:/Homebrew/homebrew-core/actions/runs/10500178069/job/29133091332?pr=180185

We should only activate a runner when there are deleted formulae if it
isn't a dependent matrix.