rust: set MacOS env variables only on macOS

[iMichka]

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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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[cho-m]

It looks like most failures are expected, so may want to check to see if safe to merge
The failures I haven't taken a look at:

• openimageio
• graph-tool - errors are about boost

The failures that are maybe safe to ignore:

• Expected failures asuka, vice, gifski, ktmpl, wasm-pack based on previous update rust 1.51.0 #73924 and upcoming update rust 1.52.1 #76780 (comment)
• Also expected failures cargo-watch, pdfpc, freeswitch
• gtkspell3 - Fixed in gtkspell3: add missing build dep gtk-doc and license #78388
• pc6001vx - C++20 <version> issue
• cargo-audit - upstream repo merge has broken source tar URL
• badtouch (authoscope) - SHA mismatch, probably fixed in authoscope 0.8.0 #78138

Once this PR is merged, we can try continuing #76780 after rebase and some fixes

[carlocab]

Changes included in #76780.