Specify DSTROOT and INSTALL_PATH for xcodebuild explicitly

Hey :)

It seems that some plugin projects do not set `DSTROOT` as `$(HOME)` and `INSTALL_PATH` as `/Library/Application Support/Developer/Shared/Xcode/Plug-ins` (for instance, CodePilot). Bundles therefore do not go where they should and installation fails. 

I think it would be best to pass these settings to xcodebuild so we wouldn't need to rely upon anything. I also added `install` build action, it seems to be the proper way to trigger installation actions (setting permissions, copying products, etc.).