on_macos/on_linux block: fix rubocop for special cases

[SeekingMeaning]

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---

These should not be allowed:

• Single SendNode that isn't depends_on/patch/resource/etc.

on_macos do
  desc "Collection of Linux utilities"
end

• BlockNode that isn't depends_on/patch/resource/etc.

on_macos do
  bottle do
    cellar :any
  end

  depends_on "libcap"
end

Before:

% brew style util-linux

1 file inspected, no offenses detected

After:

% brew style util-linux
Library/Taps/homebrew/homebrew-core/Formula/util-linux.rb:19:3: C: on_macos can only include depends_on, patch and resource nodes.
  on_macos do ...
  ^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense auto-correctable

[BrewTestBot]

Review period will end on 2020-12-09 at 22:39:55 UTC.

[reitermarkus]

  on_macos do ...
  ^^^^^^^^^^^

Would be better to actually mark the node inside the block that is not allowed.

[BrewTestBot]

Review period ended.