`assemble.all`, when run on mac, produces bad `.app` folder (but works when run on linux)

Not sure why, but running `gradlew assemble.all` has this behavior:
- Windows can build output for all platforms, except it clobbers the `executable` flags for mac/linux.
- Linux can build for all platforms (Linux CI server ftw!)
- Mac produces incorrect output for its own platform (!!!)

So you can't build mac artifacts on a mac. Luckily, you can build mac artifacts on a linux box, and they'll run fine on a mac. But it'd be nice if mac could build itself.

Likely to be a bug in PDE build, perhaps related to [this](https://bugs.eclipse.org/bugs/show_bug.cgi?id=468131)?