github_runner_matrix: filter incompatible testing formulae

[carlocab]

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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.

TODO: Update some tests.

Footnotes

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

[carlocab]

For ease of review: there shouldn't be any changes in logic here. I'm just keeping data that we used to throw away and then adding some of it to the matrix JSON.

[MikeMcQuaid]

Makes sense once 🟢! Merge at the beginning of your day so you can be around to 🔨 any regressions.