ENV/super: add Python's `libexec/"bin"` directory when applicable

[carlocab]

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When Homebrew/homebrew-core#107517 is merged, builds will no longer be able to find [email protected] as python3. This is also what is likely to happen to [email protected] when we add a [email protected].

This is likely to break many builds, so let's make sure they can keep finding a python3 for formulae that don't have a dependency on the latest Python3.

This is arguably something we should've done earlier: it also means that builds that go looking for an unversioned python end up finding our Python3 (whenever present in the build environment) instead of, say, /usr/bin/python which is typically Python2.

[BrewTestBot]

Review period will end on 2022-08-11 at 14:57:18 UTC.

[BrewTestBot]

Review period ended.