cask: ignore minimum macOS in on_os blocks for variations

[EricFromCanada]

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The fixes I made for variations generation in #17386 didn't account for when depends_on macos: is called inside an on_os block. Affected casks are set to be fixed in Homebrew/homebrew-cask#176710; until then, any cask with a minimum macOS set anywhere will not show versions older than that.

For example, macupdater is shown as requiring Ventura despite defining an older version in an on_monterey :or_older block.

It was recently pointed out that this causes affected casks to be incorrectly shown as upgradeable on older systems.

This PR revises the variations generation fix to ignore depends_on stanzas that are set within on_os blocks. This'll briefly bring back some of the issues with formulae.brew.sh pages until the affected casks are fixed.

[Bo98]

Thank you!

[MikeMcQuaid]

Thanks @EricFromCanada!