add bottle support for macos mojave and high sierra

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attempt to add CI support for bottling dependencies for macos mojave and high sierra.

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Homebrew/discussions#1970

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https://docs.github.com/en/actions/learn-github-actions/environment-variables

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following the above discussion and adding the below line to the tests.yml in this repo,

echo HOMEBREW_DEVELOPER=1 >> $GITHUB_ENV

enabling HOMEBREW_DEVELOPER allows the self-hosted runner specifically mojave to build a bottle for swig, but more importantly build a bottle for an officially unsupported homebrew version of macos. ie. mojave, as macos homebrew only officially supports the last three versions of macos.

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time permitting ⌚️ begin to setup macos high sierra vm to begin building and testing software on macos high sierra 10.13.

macos high sierra particularly interests me because it's the last version of macos that supports the official nvidia web drivers for apple hardware that use nvidia graphics cards. so i'm interested in knowing if freecad performance would improve on a machine running high sierra with the nvidia web drivers installed on it. 🤔