Build mac dmg in Action (#869)
Build mac dmg in Action (#869)

Summary:
`build --mac` no longer implies building the DMG so the upload step currently fails.
Pull Request resolved: https://github.com/facebook/flipper/pull/869

Reviewed By: mweststrate

Differential Revision: D20283002

Pulled By: passy

fbshipit-source-id: c255ae3a7e4e006e198b79ad8ce79cfd5e80b6a3

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