Update to swift preview 3
* We can't use system anymore in preview 3. This introduces a lot of issues around envrionment variables, which can't be listed in swift :-(. So we no longer inherit environment variables, we only set the ones we set. * We do pass on PWD and PATH, because otherwise that would be terrible * Toolchain is now a part of platform config instead of something we pass around by hand. * We now detect xcode 7 / 8 depending on whether we're using a toolchain installed to Xcode.app or Xcode-beta.app. That's still not right, but fuck it.
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