formula: reject only the latest head keg when cleaning up

[Rylan12]

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Follow up to #11438. See #11438 (comment)

Versions can be compared to each other using so using .sort_by(&:version) works fine. HeadVersions, though, cannot be compared in this way. Instead, the latest version is found by checking the time the tab was modified (done in Formula#latest_head_version).

Since .sort_by(&:version) on and array of HEAD kegs just returns the same array (of which all items except for the last were selected for cleanup), it is arbitrary which HEAD version doesn't get selected for cleanup.

Instead of sorting the HEAD keg array, I've chosen to just remove the latest HEAD keg as found using Formula#latest_head_version. This is essentially the same behavior that existed prior to #11438.

Here's the net diff of #11438 and this PR:

        eligible_kegs = if head? && (head_prefix = latest_head_prefix)
-         installed_kegs - [Keg.new(head_prefix)]
+         head, stable = installed_kegs.partition { |k| k.version.head? }
+         # Remove newest head and stable kegs
+         head - [Keg.new(head_prefix)] + stable.sort_by(&:version).slice(0...-1)
        else

[BrewTestBot]

Review period skipped due to critical label.

[carlocab]

Makes sense to me. Thanks @Rylan12!