formula: activate env extensions for postinstall

[SeekingMeaning]

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This allows ENV.prepend_path calls during postinstall

Another way to do this would be:

ENV.extend SharedEnvExtension

Related: Homebrew/homebrew-core#66722, Homebrew/homebrew-core#66910

[BrewTestBot]

Review period will end on 2020-12-21 at 20:41:57 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @SeekingMeaning!