shims/mac/super/xcrun: use Homebrew's chosen SDK and DEVELOPER_DIR

[Bo98]

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Fixes #7250.

Turns out xcrun is really awkward as it has so many uses. You don't want to set DEVELOPER_DIR all the time because, for example, we have an older compiler in the CLT on Mojave CI nodes - and that compiler might be found via xcrun --find clang.

[Bo98]

I'll just pretend I didn't just see Qt's odd use of xcrun --sdk macosx --find clang. I don't think it's worth overcomplicating this for that - if Qt is awkward and chooses the CLT compiler then so be it I say. It won't cause any actual harm and is really not the focus of this fix which is about xcrun --sdk macosx --show-sdk-path.

[MikeMcQuaid]

Makes sense, thanks @Bo98!